The trouble with tuning a cos is its a question of where do you stop, not where do you start However, being sensible and choosing parts for function rarther than form will go a long way to saving the £££.

If you are budgeting 10k, then think about where you want your car to excel. If you want a general all rounder, you will have to compromise the car somewhere.

The hard decsision is the engine, is 400BHP as much as you want? If it is, I would stick with a std engine to keep costs down, maybe just slightly low comp, nothing clever, maybe a BD10/BD14 inlet cam, std exhaust, and a Good T34.63. this will see 400bhp/ 400Lbs/ft if mapped correctly ( I would use MSD for this as they are the bollocks ). It will also drive really nicely and be reliable if built correctly. Also, if you want somthing a bit trick, get a marram shafted T34, and get ALS put on, top toy and not too much more expensive if you do it all in one go.

The BIGGEST investment, and IMHO needs doing as a priority is to get a decent gearbox in there, I am personally going for a "mullet" Bara motorsport box, as Quaife big tooths are a waste of time , and the fixit kits (IMHO) are going to be hard to get bits for in the future.

What is the point in having 400BHP if you are scared its going to throw the box to bits everytime you put your foot down? . So you are looking at £3500 ish for that depending on who you go to for it, but by the time you have mullered 3 or 4 std boxes( which you will do), it has paid for itself and the car hasn't pissed you off and got you stranded/ embarrased you by breaking infront of the birds )

Brakes- Ap 330mm 4 Pots are around £1200ish, and are easily one of the best upgrades you will make. People (including me ) buy 6 pots for about an extra £400ish , but they offer no real world performance advantage over the 4's, just look better

Suspension- Konis and poly bushing with decent springs is about £600- will be fine for 95% of what you want the car to do if its a road car
Can go coilovers, but staring to get into £1000's and will make the car a bit harsh for general use IMHO...

Front diff- Another weak point. I have bought a ford thickwall 909 (similar to a quaife thickwall casing) front diff, but kept the std open diff, as people complain that the ATB makes the car understeer. At the 400-440BHP I hope to achive, I might be over-engineering it a bit, but I want reliability, so have paid the money hopefully as an insurance policy.
A lot of people run a std diff and have no problems, or treat it as a wreck and replace item as they are so cheap ( normally about £50-80!) just a pain to keep swapping IMHO. You could run a steel sandwich plate that a few people seem to have had good results with, and at £40 it's got to be worth a go.

Rear beam and Adjustable TCA's are a good idea, I use Compbrake TCA's, they are really beefy and at around £150 not a bad price. Rear beams make a huge difference apparently, I have not changed mine yet, but Zoo Motorsport do a £550 road beam, or a strapped (stronger) item for £675, both of which I am considering. Also a lot of rally teams stick a 7.5" rear 2wd diff in for a bit of extra strength, works fine with std 4x4 running gear (according to Mike R ).

Then most of it is down to personal preferance, 500 coolers, underbonnet stuff etc etc.

I will post my proposed spec up a bit later if anyone wants me to, as I am almost ready to start my build now as well