I've mostly been into handguns but it's time to step up to a shotgun. So I'm looking for some advice. Right now I'm leaning towards the Mossberg 500 because it's within my budget range. Eventually I would like to do some upgrades like a picatinny rail, strap, and shell holder. Is there anything I should know about doing these upgrades? Any recommendation on brands or types? Also anything I should be looking when shopping for the shotgun? I know there are a bunch of different 500 models so I'm not really sure which one to get. I know I definitely want a 12 gauge with a 18.5" barrel. When examining the gun anything I should be sure to look out for? Thanks in advance your help.