1999 Volkswagen Beetle AC blows warm

Normally your mechanic will charge the system with refrigerant, then test for leaks, but at the age you listed, it is common for most of it to have leaked out and a simple recharge will solve the problem. I'm guessing you used a do-it-yourself kit and recharged the system yourself. In most states it is illegal for mechanics to not test for leaks and to reclaim the refrigerant if a leak is found.

Regardless, the original problem started with the leaking evaporator and the fact it wasn't discovered right away has no affect on the solution. The problem now is replacing an evaporator on any vehicle is rarely a do-it-yourself project. If you think you want to attempt the job, get a copy of the manufacturer's service manual and read through the many pages of instructions. Don't use Haynes-type books. They only cover the typical do-it-yourself stuff, and even that isn't as detailed as it is in the manufacturer's books.