Warhammer

I have a 2003 with the 5x135 bolt pattern, and I want to go to 20" rims. Well, the selection of rims in this bolt pattern is thin to say the least. I'm thinking of getting some adapters so I can have some more options to choose from, but I have a few questions:

1. Are there issues I need to be aware of with wheel adapters?
2. Do it need to compensate for the adapters with backspacing?
3. Which bolt pattern would be best to go with?