Sorry if this is old ground - couldn't be arsed to read it all, all this is in my opinion of course from knowledge of Karls car from when he got it ans subsiquiently when TIff had it and also from lookin into buying one.
1.6 gti or mi16 cars converted from 1.6's- AVOID- majority of the running gear isn't compatable with the upgraded bits you can buy- such as 307 brakes and its a 1.6.
1.9gti personally I wouldn't pay more than 1k tops- with resonable history, exhaust, decent tyres, standard rims and evidnence of some engine work- paying more gets you a mint car yes - but do you want it because its a collectors piece?? I think not. When you consider the 1.9 Tim has cost him LESS than 1k with a cage!!
Mi16 cars- a mine field and a timebomb waiting to happen- the correct car should have the proper wiring and engine management (ie no dizzy) and be mounted at the correct angle- but what is the correct angle?
So many people do so many different things- from having a look previously sometimes I think its better to go with a 1.9 to be safe.
Price wise- 2k tops for a good solid car with a good engine converted properly- the power steering rack is a quick rack so make sure its still in there. 3k should see a car with cage/buckets/harnesses/brake conversion maybe in a nice colour and maybe with some other trick bits like a 309 rear end- 309 front wishebones and a solid mounted rear beam.
Personally - given the choice I'd have a 306 gti6- No problems with the oil issues, engine already has reasonable performance, handles like a dream, proper brakes already fitted, readily available and cheap enough if you look hard enough.
Failing that- 306 xsi 2.0 16v - mines ace- only down side compared to Gti is the brakes, 6gears and 50 less horses
Jake