My dad and I go riding a lot. We have a group of about 20-25 guys that we go trail riding with. We normally go to Chadwick here in missouri, which is a perfect trail spot. They have rated trails for easier riding or extreme riding. We have a 750 Kawasaki Brute Force, and that thing is powerful. They're really good for trail riding, but they're really big. My little brother has a Honda 400EX and it's a really good bike. It's a sport bike with the manual w/ a clutch. We also have a Honda Foreman 500. It's not fast, but that thing has lasted through everything. It literally got drowned in a river, and we drained the oil, and it still runs like new. As far as helmets, don't get cheap. You're gonna appreciate it when you fall off and land on your back. I'm glad I had a helmet at that point
As far as trailers, that's up to your needs. We normally get 3 goosenecks for the amount of bikes we have. But if it's just you and another person, you can find a good smaller trailer and drive them up the side with a set of ramps.