Right on the computer now so essay time!
Frame: For your budget, its essential you go second hand. A good full sus frame is easily 1k plus. Why do you need a full sus though? A good hard tail with 130-140mm of fork travel will be very capable over some very rough trails and you can get a lot more for your budget. In fact your budget isnt really big enough for a decent full sus but if you have your heart set on it, go for no more than 6" rear travel otherwise you will really struggle to ride anywhere except down hill. Also dont forget with a full sus you have to factor in extra servicing costs for the shock as most units now aren't really home serviceable.
If you go for a hard tail, go brand new. I always prefer to go brand new because you dont know how the frame was treated if its second hand so could possibly have stress fractures etc which can be almost impossible to spot until its too late. For hard tail, id recommend dialled bikes. Frames look very basic however they are extremely well built and stupidly light. I know the guy who runs the company and he spent about 4 years developing the prince albert frame in the surrey hills. Ive had the mk1 and mk2 and now have a mk3 of the prince albert and the mk1 was good but you wont believe how much nicer the mk3 is and how much lighter!
Forks: Again, dont be fooled by a cheap fork that has loads of toys. Not worth it and normally they are shit. I personally love Rock shox and Marzocchi but if you can stretch your budget, get some fox's! Ive got a set of 11 year old marzocchi 150mm travel air forks and they are still silky smooth. Only issue is the bushes are shot but thats a common problem with that model.
Travel is obviously dependant on your frame but for a good all rounder, id recommend 130-140mm travel. Most 5 or 6 year old and newer forks have a lock out of some description as standard now so dont worry about that. If i was you, i would be putting about £200 of that budget towards a second hand set. Bargains pop up on ebay all the time and if your lucky, you might grab a set of fox vanillas for that. Air forks are generally the most expensive for servicing. If the air chambers fail, they are generally a sealed unit so you have to replace the entire chamber instead of just a seal for example however air forks give the nicest ride and generally the most adjustability to your weight and preference. Oil changes are simple to do at home your self and cost about £10 for the oil and should be done about every 12-15 hours of riding (not as often as it sounds!) If you go for some rock shox spring forks, be aware they are normally very soft so be prepared to upgrade the springs although they are normally only about £30 for a set.
Brakes: Dont skimp! You want disc on both front and back but dont get cable what ever you do,. You will spend more time messing with them than riding them. Once again id go second hand. You find with cheap brakes they are very on-off and not very compliant and also dont do well in tough situations. My personal favourites are Hopes. For a start, they are generally a work of art and secondly, they just dont fail but because of this they do fetch a premium. I paid £100 for my set of 5 year old 4 pots and that was considered cheap. A full rebuild on a hope 4 pot will cost about £30 a calliper but i doubt you would ever need to do this. Pads, just stick with standard. EBC do pads for nearly all bike brakes (and i think brembo do as well) and they work very well but you can easily demolish a set in one hard ride which is not what you want at about £25 a set. Standard pads are normally best. When buying, be careful as there is plenty of mount options depending on your frame/forks. Fluid changes should be done again about every 15-20 hours but they just use standard dot4 or 5.1
Also a good upgrade is Goodridge hoses. Make the world of difference and they come in pretty colours. They are about £25 a brake so maybe something to consider later down the line.
Also, Ed will be jealous you have 4 pots
Gearing: Personal opinion is stick to Sram. I always seem to have adjustment issues with Shitmano but ive always found even with the cheap X3 range that sram do, its just a case of fit, adjust, forget. Dont be tempted to buy a Shimano shifter and sram mech because they wont work. up until a few years back, sram used to work on a 1:1 ratio and shitmano on a 2:1. Think it was only a few years ago sram started offering 2:1 bit but at a premium.
The budget sram range is the X3 and i think they are very good. I always tend to go with a x7 shifter and x3 mechs because i have a habit of smashing them up on rocks and they are cheaper lol. Thats brings me back to frames. If you go for a ally frame, make sure it has a removable mech hanger. If you catch your mech and rip the hanger off, the frame is basically f*cked unless you know someone who can make you a new drop out and weld it for you.
Wheels: Avoid shimano hubs and rims like the plauge. Shimano hubs just fall to pieces and have a habbit of taking the housings with them so you have to replace the whole thing. I would go for hope pro 2 hubs. They are expensive but bomb proof! Older ones had a ally cassette body and you would find sometimes you could chew it up with hard riding needing a new cassette body but if you get a newer one, they are steel so not a problem. Also, the rear ones sound like someone rattling off a machine gun! Epic
Rims, im a big lover of Mavic. Nice and cheap and generally very good. Which ones depends on your style of riding. 729s are absolutely bomb proof (and one of the widest rims about now) but are bloody heavy and cost about £60 each! 719s are a good all rounder. Light, not as wide but still strong. I have them on my current bike and my last one and never buckled one and im a fairly heavy handed rider and always seem to end up landing on logs and roots lol.
Be careful who builds your wheels. CRC are not very good at building wheels im told and this is where a good wheel builder is worth his weight in gold! A badly built wheel will buckle and fall to pieces in a few months. My old wheels were built by Merlin cycles who are very reputable and i happen to know that even though they are now 7 years old (f*ck that makes me feel old!) they are still going strong and true.
I wont lecture you about tyres because much like a car, that is very much down to personal opinion.
Everything else: Much down to how much cash you have left. Buy all of the above first and prioritise them because they are what makes a bike a bit special. Personally, im a brand whore so anything with hope written on it, i will have if i can afford it. Its generally very very well made and looks porn. If your budget is even bigger, go for Chris King but you will have no hope on your budget (but for Lolz, google the Chris king rasta head set. I will be buying one!).
Dont underestimate ebay for buying these bits. Bits that can be a month old can come up very very cheap on there! Also, keep an eye on CRC. They always have sales and if you dig deep, you will pick up some serious bargain. I know when Grant built his last bike, he picked up a £250 set of race face cranks for about £30. Its all about digging on that site!
Finally, once its built, come with us to chicksands!
Hope this helps a bit!