Joey, welcome to Ferrari Life!
1. Depends on which 308 model you are after and in which country. Early fiberglass carb 308 GTBs are worth 2x what a 308 GTBi would run.
...in England, I suppose. This statement definitely does depend upon which country. Here (U.S.), a euro GTBi QV is just as rare and desirable as a fiberglass carb 308 (because of weight due to emissions standards, as well as body stying). Only 712 fiberglass 308s were made and only 748 GTBi QVs made. I chose my rare euro GTBi QV over a fiberglass (by the same seller, at the same price) back when the U.S. economy was still strong. Of course, the U.S. saw a huge drop in exotic and collector car pricing since then and only recently have prices started to come back up. Still, you'd be hard-pressed to find a good example of any 308 for less than U.S.$30G that didn't need money sunk into it.
2. No values move with the market based on supply and demand. All are full depreciated. Values in the UK currently range from GBP 40-45k for a Fiberglass 308 GTB to under GBP 20k for a 308GTBi.
U.S. figures are the same, but the statement should end with "GTS or GTB" not "GTBi" since the european fuel injected GTBi is on par with the same cost as the Fiberglass 308s (which were all GTB models). Again, l'm talking about the U.S. market only.
3. No there are plenty of knowledgable mechanics around. Never had an issue finding parts.
The same goes here for the U.S. With a 308, you probably have the easiest Ferrari to maintain.
Joey, I hope you are settling for the car and color that YOU want, and not depending on our opinions. For instance, I wouldn't want a 328 over a 308, because I like the toggle switches and black bumpers. I wouldn't want a targa over a GTB, because I like the shape, side windows, and rarity of the GTB. I wouldn't want any other color but red, because I find it exciting on a Ferrari, and I like the red racing heritage. Others feel the same way about their cars which are different from mine. You have to decide what you want and go from there. Don't buy anything other than what you want. And when you do, post pictures!