Not all V3 gearboxes work in a G36. There are different versions of V3 for AKs, AUG, G36. MP5K, etc. While they're pretty much the same on the inside, they have parts like the trigger, selector and nozzles that are different. So you have to get a gearbox specifically for the G36.
As for it being "old" and "worn", that to me sounds like a cop out from someone who doesn't know what he's doing. Unless the mechbox shell is physically cracked, which would surprise me with a reinforced CA V3 shell, then there's no reason to replace it. Some internals may be worn or broken, causing your problems. But installing a "whole new complete mechbox" is usually the easy (and most expensive) copout when you just don't know what's wrong.
I'd say to get someone who actually know what they're going take a look at it, and I bet they'll be able to fix it without replacing all the internals. A lack of distance while shooting may just be the hopup rubber that needs replacing and that's about $10.