Wow, you have a lot of questions. I believe you will find the answers to your questions by cruising through the C3 forum. There is a lot of "been there, done it" info for you to learn from.
Long story short, check the local emissions laws where you live. If they test cars going back to 1975, then it won't do you any good to pick up a cheap low power 77 and drop a crate motor in.
If you don't have any regular specific testing, you might get away with it.
I think you will find others here agree that buying a crate engine with matched components will yield the most predictable dependable results.
Short of flywheel changes (newer/older blocks), and some other small mods, there is nothing any different about dropping a 400 hsp crate engine in instead of a stock replacement.
Remember to, that a good crate motor with headers, exhause work, pulley change, carb, high output ignition, dress up accessories, etc can run you $5-$8,000 alone on top of the purchase price. If you put all this in a 77 model, you may never get your money back out of it.
So, back to the 68-72 models that already have high output engines, and no emissions inspections.
If you are diligent in your search, you may find a good complete car that somebody has to sell due to divorce, death, etc. and you can get a complete package. It is easier to finance the entire car ready to go the way you want it, than to buy the car, then piece it together here and there. That is how you destroy the old credit cards.
Good luck in your search!
BTW, you could always get a low output super clean 75-82, then strap on a 250 hsp shot of NOS. That is the best of both worlds. That once in a while need you have for speed can be fulfilled with a push of a button, while retaining the clean, easy driving of a stocker!
