Let me ask you a few things:
1: What is your old/current transmission doing that leads you (or the diagnosing shop) to believe it needs a rebuild?
2: Are you more concerned with money or performance?
3: What scares you about rebuilding the C4?
If you are on a budget and have found someone to sell you a rebuild trans
for $350, I say go for it. The rebuild kit will run about $100 for the 'master rebuild kit'. $950 sounds a bit high to me. Have you tried a good mechanic or machine shop? One with old gray-haired guys hanging around?
Transmission shops can be entertaining. I had an old minivan that was popping out of gear. I took it to a chain trans
shop, and they said "Yep, needs a rebuild". I took it to a mechanic in a hole-in-the-wall shop, and he told me my linkage was out of adjustment. Showed me how to adjust it, and charged me $15. Vs around $1,000 for a rebuild!
If your car is not shifting well, or not at all, or slipping, it could be fluid level, vacuum lines, gaskets around servos.....I would sure try a lot of tricks before rebuilding. I had my C4 (66 C-Code) rebuilt about 10 years ago for around $350. That included the labor. The only reason I had it rebuilt, was the front seal was leaking like crazy, and I had a nice cloud of smoke following me around. My mechanic said it would cost just $35 more to rebuild the damn thing.
I think if you are quite comfortable working on your car, are patient, and have some good tools, you could rebuild it yourself. However, the rebuild kit is
$100, so if you goof up and end up needing a pro, you just lost $100. Something to think about.
So let us know what's going on with the car, and maybe we can help you out and save you some trouble and expense!