don't ever waste your money on the 4 stroke sleds (yamaha). they don't work unless you dump an extra $10,000 into them.
the best cheap sled you can get is the 98 and 99 skidoo summit 670x. before 98 their high altitude compensator sucked. 98 they switched to dpm, and it made them awesome. they can use a few updates, (new skis and handlebar risers are essential, plus the seat is very low), but they are light, ultra reliable, and parts are cheap. here in bc you can pick one up for around $2500cdn.
the 99 polaris rmk 700 was good, but parts are expensive if you ever have to fix them. the early polaris 800's were garbage, and the used price will reflect that. they had it straightened out by 03 though. don't go near the arctic cats prior to 05. the yamaha 2 strokes of that era were good, but their tripple's (3cyl) were pigs on fuel.
the 00-03 skidoo zx chassis summits are awesome, but they are not without their problems. their top ends wear out quick. here is an article describing the problem, and the fix if you are willing to spend the money
http://www.2strokeheads.com/tech6.htm
the rev chassis doo's are great, but they are more expensive. they hold their value really well.
if you have a little more money to play with, the best bang for your buck used sled is the 05-06 arctic cat m7 efi with 153" track. they have wicked power, they go anywhere, and the motor is efficient and reliable. the seat is a little low, but its long which is great for doubling. they only require a couple inexpensive mods to make them work right. extra venting for the clutches is essential, and if you get one with the attack 20 track, it must be turned in the hillclimb position if you expect to go anywhere. thats about it. the efi makes them very easy to start, and they never get flooded like a carbed motor will. they are designed to run on 87 octane fuel, unlike all the others which require 91. there is also lots of aftermarket support for the m chassis cats if you like to tinker. i've seen these sleds for sale in anywhere from the $4000-$6500 range. the only thing i don't like about the m's is the fuel gauge. it reads empty once it reachs half a tank. it feels a little scary at first, but in reality there is always lots of fuel in there.
in general, track length determines how much terrain you can access. a short 136" track will get you into the backcountry sure, but its tough to double up the steep stuff. hard to keep the skis on the ground. and you wont be able to climb too high in the pow. thats not to say that a 136 wont work at all. if you are on a budget, a 136 track can be considered to be the bare minimum. in fact, if you get good with it you will be a better rider.
handlebar position is probably the most important issue to address on any sled. in most cases you need to raise your bars up, and pivot them forward. this gives you much more leverage for throwing the sled around, it allows you to stand straight up, and it centers your riding position closer to the front of the sled. being closer to the front gives you a smoother ride over the bumps, and it gives your passenger more seat to sit on. speaking of seats, a taller seat makes life easier for your legs because there is less travel distance when going from sitting to standing.
all sleds are hard on fuel, but unless you are riding more than a total of 100kms/60mi in a day, you should never need to pack extra fuel. the most i've ever done in one day was 117kms with still some to spare. average is more like 65-70kms
the 670x is the smallest motor that you should consider. other than that, you want a 700 or 800.
if you have any specific questions about specific sleds, don't hesitate to ask. i can most likely help you out.
[Edited by polemite.productions.dave on 11/13/2008 at 11:35 PM]
[Edited by polemite.productions.dave on 11/13/2008 at 11:42 PM]