2 choices, Bis or aircooled, Bis tends to be easier to find, is watercooled watch out for cooling/ head problems.
Aircooled, a real 126 (only joking) A bit slower and tempremental than a Bis, but .......... Never mind
Look out for rust underneath, inner wings!!! It will have runt at the bottom of the screen inless you are very lucky, and ,most have a rusty spare wheel well in the front, and some rust around the lights.
They are not like a modern car to drive, they bounce around a lot and rattle and shake. spares are cheap, running costs low, but this is not a modern car, not a Micra or a Smart.
Have I put you off yet? Prices watch Ebay! £300 up to £8000 if you want an as new rally car.
Good luck