I had a Belarus 420 AN for several years. I would say that it is a mixed bag at best, but could be a good deal.
The electrical system was extremely primitive and poor - I ended up replacing the original alternator (generator) with a normal self-exciting one and changed out most of the wiring, fuses, switches, etc. Took a while, but well worth it.
Most of the hydraulic system leaked, and if I remember right, it used motor oil instead of hydraulic fluid (30 weight or something, I think).
It took two good batteries to start it up, but it did start well in cold weather as long as the glow plug/manifold heater worked.
The engine never leaked a drop of oil, or burned it, but it was L-O-U-D. I remember that most of all. I changed to a synthetic oil and that helped with starting and it actually seemed to run quieter, but that may have been my imagination.
I don't know if parts are difficult to come by now or not. At the time I owned mine, they were easy to get but pricey.
If it is less than $2500 or $3000 and runs, it might be a good deal, other than that I don't know if I could recommend it.
Take care and good luck.