Track length depends on how you are going to ride. If you are like me and enjoy powder the most, then you are going to want the 150+ range. I ride a 08 Ski Doo 800 154 track, and I love it. Arctic cat builds a nice motor, but at the cost of extra weight, I have never ridden one though. The M8 seems to be a popular choice. Ski Doo's steering is different than the Polaris and the Arctic Cat. You would want to ride one first before you convert. I am used to the difference so now I can Boondock and side hill easier on my '08 xp than I can on my brothers 2012 Polaris Pro. My xp has upgraded suspension though, and I am used to throttle input on my machine vs his. Throttle control makes initiation a powder turn much easier.
The 700 does seem to be more reliable than the 800, but it doesn't have as much power either. No matter what if it has more than 2000 miles you should look into putting a top end in it. That is just the nature of two strokes.
I think if you can get into the rider forward chassis no matter which brand you will be happier and have more fun out in the pow then if you where to stick to a wedge chassis. I chose Ski Doo because they invented the rider forward style, so for the money you get the most technology. My '08 I got for $4500 after all taxes and fees, and it only has 730 miles on it.
Long story short...
1. decide a budget.
2. decide how you are going to ride (trail, powder, trees, hillclimb, jumps, etc)
3. Decide a brand that suits budget and riding style.
For me that meant Ski Doo.