If you go with a four stroke, miles aren't a big concern. The motors usually last for 20k miles or more. If it's an older two stroke tripple (three cylinders) it will likely run for a long time as well. I've known guys that have put 10k miles on Yamaha SRX's and Mountain Max's with no maintenance to the engine. But most of the older tripples have been retired or are just old and beaten.
As a general rule, most two cylinder two stroke motors need a rebuild at 2500 to 3000 miles unless they have been babied. The exception is the Ski Doo ETec motors. They have been around for five years of so and seem to have good longevity. I've seen guys online saying that they have put 5K miles a year on them with no proplems. The guys that I ride with that have them have been very happy, but I have read complaints that they don't have the power that their predecessors did. To me, I'd sacrifice some power for reliability, but some people value power more.
As far as what to look for? It sounds like you are looking for a trail sled. If you want a 2 up, that's about all you're going to find. If you plan to buy new, any of the manufacturers are a good choice. Ski Doo, Yamaha, Arctic Cat and Polaris are making good stuff these days and I don't think you can go wrong with any of them. I'd recommend going to the dealers, telling them what you're looking for and see what they offer.
I know I'm going to start a brand argument here (some people are really sensitive about the brand loyalty thing. If you don't ride what they ride and acknowledge their superiority, you're a dumbass) but Arctic Cat and Yamaha are using the same Yamaha four stroke motors now and they are arguably the most reliable motors in the sled market. And Yamaha has two other motors (a 500 cc in the Phaser and Venture and a 1000 cc in the Apex and Viking) that are very reliable as well. So, if you are looking for a sled that you won't have to do any maintenance on, I would recommend a Yamaha or Arctic Cat four stroke. I have one that's 13 years old with over 7K hard mountain miles on it (and expect to get 15K more) and haven't had any motor troubles. And that is to be expected with them. I've had to replace some wear items and had to repair as a result of crashes, but otherwise, it's been solid. I'm 50 now and expect I'll be too old to ride anymore by the time my sled is needs to be replaced. The trade off is that they're heavier which is an issue when you get one stuck. But they're all heavy when they're stuck.
Another thing I'd recommend is getting some time in with an experienced rider. Surprisingly, there's a lot more to it than squeezing a throttle and turning the handlebars. Everyone's a dumbass when they first start riding, and getting some pointers from someone who has some experience is valuable.
It's also a bad idea to ride alone. I've done it myself and probably will again, but I have many stories about acquaintances who have nearly killed themselves doing it. Just last season, a guy I know had to get hoisted out of a ravine by a Coast Guard helicopter, and he had two riding partners. But he took off on a little side trip and they didn't know where he had gone. The other two finally called 911 after not being able to find him for a couple of hours. And he was less than a half mile away. If the Troopers and Coast Guard wouldn't have found him (about 8 hours later) he probably would have died in that ravine. He got into a situation that he wasn't prepared for and it could well have killed him. Things can go wrong very quickly, so it's a good idea to have someone riding with you.
Another thing I would recommend is talking to your neighbors. See what they are using and pick their brains for information as to why they are riding what they're riding. If everyone there is riding a mountain sled, or a utility sled, there may be a reason.
The last thing I'll leave you with is my preference for mountain sleds. I have been able to make a mountain sled work on trails, but a trail sled doesn't usually work very well in the backcountry. I know people back east prefer trail sleds, but there are a lot of groomed trails back east. Many of them never leave the groomed trails, so it works for them. If you're going to be breaking trail on old logging roads in 10 inches of powder, it may get real old real quick on a trail sled. The mountain tracks are longer, have bigger paddles and mountain sleds have wider skis to deal with those conditions. The only thing you really need on a mountain sled to use it on trails, rivers and lakes are ice scratchers in order to keep the rails lubed and keep the sled cool.
I really could go on forever, but I think that's a good start. Good luck. I hope you find the right sled for what you're doing.
