Well first off if you're even considering buying a GT then I say just go for that because there is no sense putting parts into this car only to sell it a few months later.

4.10s should be fine. I definitely wouldn't go any higher. The higher you go the more rpms you'll be turning at any given speed, so for example right now in top gear you might be doing 1500 rpms at 60mph but the 4.10s will increase that to around 2000 rpms which would result in a slight decrease in fuel mileage and a louder interior.

You don't have to change anything in the transmission but you will need a speed calibrator to compensate for the different gear ratio otherwise your speedo will be off.

Changing rear end bearings is up to you. When I was calling around about getting gears changed some shops wouldn't do the swap without changing the bearings as well because they didnt want to be liable for the bearings going bad after everything was said and done. Its really not that much more so you might want to just get them done while everything is apart.

When I was looking to get the gears done in one of my cars prices varied quite a bit from shop to shop but that is mostly due to the different brands of the parts they're installing and and what was included. $500-$900 with everything.

BTW Your car has a 7.5" rear end which is different than the 8.8" rear in the GT. You might want to point that out to whatever shop is doing the work so they get the right parts.

Honestly, I'd save the money for a GT.