The bearings used on the wheels are "standard" in-line skate sized. The other bearings can be found almost as easily. I buy all my bearings from an eBay reseller who stocks all the sizes. You could probably get the bearings for less than $50 (not sure what quality you've found or are looking for).

I forgot that I do have an FG alloy spur gear carrier holding a plastic spur gear. As I mentioned above the steel pinion works well with a plastic spur. I run a fairly worn plastic spur on my buggy for a couple of years now with no signs of further wear after going to the steel pinion.

I'd recommend getting the broken parts sorted out first and see what else pops up. The stock servos may work fine. I run a stock Futaba S3003 for throttle/brake. It's a piece of crap nylon geared servo that is somewhat underpowered for the stock brakes but I still have fun running the buggy.

Let us know your progress as you fix it up.