Tim's right, Start with an older car and then use the considerable sum of money you've saved to up spec to what you want.
I'm doing that on my 4/4 and depending on age the only difference is 2 extra sparks and 2 extra injectors.
IMHO the main thing is to avoid cars where the dash is controlled by the ECU and other electronics as that does complicate things quite a lot. I.e an earlier car but it might be that the Roadster always had "electronic dashboards".
My Omex ECU including the basic loom only cost about £1000 all in and then after I installed the basic Omex map and tweeked it a bit it only took 3 hours on the Dyno to get a very nice drivable car that sailed through the emissions test at the MOT and returns 38 mpg with mixed driving. (And no circuit boards to keep blowing)
Now I can play with the engine tuning and dashboard to my hearts content (or pocket allows) and simply adjust the ECU to suit.
