Mark....Regarding your questions:
1. I have used both metal and plastic shielded bearings in glow engines on planes.
2. There are many different kinds of bearings of various quality out there.
www.rcbearings.com is a good place to get glow engine bearings, and he ships world-wide. Maybe the engine experts can add more knowledge here on bearing types we need to use on glow engines.
3. Normally it is a good idea to replace both since you have the engine apart, but I recently just replaced a main bearing and left the front one in place as it was fine. No issues replacing just one.