I'm slightly interested in buying HoMM 2 but first I'd like to know whether it has a random map generator or not.

Not in-game, but the map editor that comes with it has a random map generator (which has lots of customizable options).

So you can create a random map in the map editor, save it without seeing it (which is fairly easy to do), then go to the game and select that map.

There are also lots of fan made maps available, for example here:

http://www.celestialheavens.com/heroes-of-might-and-magic-ii/general/homm2-maps
http://mapy.heroes.net.pl/szukaj/25