This is my first year using sleds for backcountry snowboarding. Originally we didn't plan on much sledding but it became just as much fun, if not more fun than the board when conditions are right. My buddies all ski, they aren't into the sleds as much as I am since they're sleds are all way underpowered and not clutched great.
But as mentioned I recommend you buy something with some power and capability. I bought a 2005 king cat for 1500 bucks and have put about 1000 bucks into mods to improve handling. It climbs just as well as many modern 800cc sleds but is super heavy and takes a lot of work to sidehill and stay on edge. But its a great tandem sled since it has a 162 inch track and a 900cc motor. My buddy bought a zr600 for 500 bucks and that sled gets around great on the trails and flats great but cant climb that well with one person, let alone two. So I recommend buying something with power. Newer sleds are going to handle far better and are so much lighter but the older power sleds still hang in there for climbing straight up hills. Try to find a 160+ inch track, as mentioned you defiantly don't want a short track, you might be able to get away with a 155 on a strong motor but the bigger the better for hauling your buddies up a steel hill. I love my king cat, but next year I'm buying a newer Polaris or skidoo once I graduate college.
I'm in Oregon so we are even worse off on forest roads than Washington. We have a few spots we are learning but most of our best lines have been on the hills off the side of the roads. Find something that looks steep and deep plan on getting real stuck, the throttle will get you in and out of most trenches so don't be afraid to use it.