15-20k will certainly buy you a very decent C4. They are very undervalued at the moment, and prices can and will go only one way, UP.
Even though it sounds not long ago to people of a certain age ( including me) the late 80,s ie 1988 is 34 years ago, that’s a bloody long time in the life of a car. Cars are out there, and especially with the internet all their pitfalls are available to read about.
sourcing from the US is fine, but prices are firm as people are appreciating what these cars were/are. Bloody good cars.
keep searching your bound to find one, inspections are easily arranged both here in the UK or the USA if you find one stateside. It’s usually money well spent.
