I just bought a 2002 Arctic Cat 660 2-upper. It works great for me and I got a good deal on it for $2500. There was one thing I needed for it though and that is studs on the track. Studs are extremely important in my view so you can get traction on the ice and not be sliding everywhere. You do not need to purchase a sled with studs on it already however because you can get a track with studs for around 50 dollars.
A 660 sled is quite a bit of power for an ice fishing sled but it is nice to have. I recommend something no less than a 500 so you can make it to your destination in a fair amount of time and be able to pull heavier things behind your sled.
Hope this helped and good luck with your search!