Electric is the best all around for cost, reliability, performance, versatility.
There are HPA drop in kits, but they're fairly expensive.
Gas guns are great but tend to work in a limited temperature band. You can mod them to HPA as well, but that's really annoying.
Most guns have full auto, most older players never use it. The amount of full auto used depends entirely on the culture of the group you play with. People more concerned about accuracy and safety tend to do semi-only.
And all BBs are degradable to various extents, the bio's are just safer. Use BB bastards; very high quality, Canadian distributor, stringently tested bio BBs.
Private land is fine, but as the guns are very realistic, just make sure the RCMP knows you're there so they know how to respond to any complaint calls that might come in.