You are asking the right questions, but you may have to answer a few of them yourself before you can move forward. First and foremost is how much can/do you want to spend on a sled. If its $1,500 you can get a trail sled that will be useful. If its $3,500+ you can get a good mountain sled. Both have their advantages:
When you catch your trail sled on fire, its not that much money wasted.
Trailsleds don't overheat on trails and are better for towing your buddies.
A cheap trail sled leaves you more money for gas, a trailer, maintenance, and a ski rack.
A mountain sled opens up the options of where you can go, and where you can get really stuck!
For a mountain sled you will want a track length of at least 155, and an engine size of at least 700cc.
Sounds like you need to nail down what you want a bit more before people can help you more. Talk to friends who have sleds, see what's available on craigslist, and think about that budget. If you have any more questions please let me know!