What advice would you give someone looking to purchase a C4?

Definitely treat the C4 as a hobby.  The owner is going to have to learn as much as possible about the car (I went to the college across the river and took auto shop classes).  The technology advanced quickly (and pretty quietly) DURING the "reign" of C4 and covered 2 distinct versions of On-Board-Diagnostics technology.  Thanks to YouTube there are plenty of folks who have shared what they know about that earlier era.  You're going to have to be curious!

Don't buy a project - unless that is your thing.  Spend a little more money and get a good quality driver.  You'll be money ahead.

My wife is not a fan of ours. No room to carry  "Stuff" on a trip. I like to Be Prepared, so I carry heavy metal. She likes to bring things that won't be used. Well, my stuff doesn't get used either, but it's there if the need arises. Also, if you are of any size other than an athlete,  ingress and egress is a little difficult. I enjoy mine.