As a former Belarus 420AN owner, I can give you some firsthand insight into my experiences. The AN is merely the low-profile version of the tractor, sits lower to the ground (i.e. smaller tires).
The 420AN I had was a powerful machine, but it had it's share of problems, mostly electrical and hydraulic. I finally did have it pretty much rewired and fixed the worst of the leaks using aftermarket parts (many of the Belarus parts are not very good quality and are fairly expensive).
I'm not sure about dealer support - there is Petsch (sp) in Hopkinsville, KY that used to and I think still does deal with Belarus tractors; most of my parts came from Beshears in Alabama. I'm not sure if they are still in the Belarus business or not.
To purchase one would depend on your needs and desires. If you need an occasional powerhouse and don't mind working on leaks, electrical stuff, etc., and can get it at a bargain basement price, hey go for it. But if you want something that you can just go out, turn the key, and go, maybe steer clear.
If given the opportunity to pick one up for $1000 to $1500, I probably would, simply as a spare time tinker tractor. I would not buy one as my sole tractor (although I got by for about 6 years with one).
The air cooled engine was the best part of the tractor. In all the time that I had it it never, ever, leaked or burned one drop of engine oil, and I ran both dino and synthetic (not at the same time). It ran better on synthetic, in my experience. The guys that I talked to at the Belarus headquarters in Milwaukee (at the time) said that you can't really go by the gauge for the air cooled engines because it is the same one that they use for the water cooled engines, which run in the middle of the gauge normally. The air cooled engines peg the gauge. I got an aftermarket and it worked out alright. I can't remember the temp it ran, but I want to say it was about 250 F.
Again, just decide if you want a tractor to tinker or one to work with. The Belarus can be an excellent value if you have the time and patience. Good luck.