IMHO it is worth buying one already done but with a few caveats:
-Go over the car with a very fine toothed comb. It may run almost well enough and 'require a tune up' but IME this is also a code for 'I tried what I could to make it go better and now I have given up" or "there is more wrong with this than what meets the eye". If there is a niggling doubt I'd objectively look at what may be doing and not simple a 'best case scenario'.
What kind of quality is the conversion? Does the wiring look like something of nightmares or is it actually a tidy conversion?
-Has it been driven much? OK alot of these cars are show cars but if it has only done a 100 miles between MOTs why is this the case? Maybe I am alone here but if the car was working well I'd want to be driving it alot! I have probably put more miles on in my RST in the past year than the previous owners did over 5! I have ironed out alot of faults however and made it drive IMHO like what an RST should as you can see via this thread.
http://retrorides.proboards.com/thre...urbo-bit-essex
-What quality of parts have been fitted? There is alot of shite out there for sale these days for these cars (ignition parts, suspension bits etc.). If they have skimped on bits here where else have the costs been saved?
The above may end up with you looking at a car which looks stupidly expensive but it will most likely be cheaper in the long run on the assumption that you have done your homework. I am sure that others will say that their conversions/projects have not been cheap cars to use!
Building one yourself IMHO is the best way but I would not underestimate the costs. I know my RST cost somewhere into 5 figures to build (car, engine, turbo, suspension, brakes etc. etc.) and from previous projects I have probably spent half as much to create a well running, but poorly finished result (i.e the bodywork was left as a result of the cash going into the car itself on suspension, engine etc!).
Try not to laugh here, but a friend of mine has just built a Rover 216i Coupe using Honda VTEC parts as part of the turbocharging process, with the gearbox/engine etc. all being rebuilt and so on. Without him touching the bodywork, or suspension or even mapping the car or making the exhaust he is already well over £5k into the build... I am sure others have done it for cheaper but it is something I have rarely seen and generally not without issues (there is always an exception to the rule before someone puts me back in my box to say "I built mine for £3k and it's fine....").
That is not to say I would not buy a done car secondhand. My Porsche 944 Turbo was built prior to me buying it and it did 10,000 miles of trouble free miles for the year, and it is serving the new owner well as a daily! My Triumph Spitfire again was ready built and despite a few issues not described when I bought it the car was cheaper and building one myself and the same goes for my S1 RST, albeit it has been a little more troublesome than the above!