It looks to me like the only difference between 13's and '14s is the updated QD belt. By now, lots of '13s are on the '14 belt. The '13s are the first year that get the reinforced skirt in the cylinder.
There are some things to look for on the 13s. You need to check the bearing for the driveshaft behind the secondary clutch. Some did not press fit very well and the shaft can spin in the race damaging the driveshaft and bearing. You should look at the QD mounting plate to make sure it was glued properly and is tight up against the tunnel. You should look at the glued ends on the lower a-arms in front. Some were not glued very well. Mine have visible glue pretty much all the way around and have been holding up to my abuse wonderfully.
The engine, regardless of year, would get a compression tests and a look through the exhaust ports to check ring condition. These are notorious piston and ring wreckers. Fortunately if you catch it before it fails the repair is not too expensive.
I just don't know that reliable and snowmobile go together very well in general so... good luck with that.