They're so small that they're extremely uncomfortable for anyone over 5' tall. As far as specs, it depends upon which model.
Speed? They're slower than molasses. I can run circles around them moving dirt with my 65 year old N series tractor and my 40+ year old John Deere 210. The tracks can damage any grass that you get into, and it doesn't take much dirt in front of the blade to make it sit there and spin the tracks.
Chances are a mini dozer of that vintage, unless it has already been restored or used very little, is going to have plenty of stress cracks. Every one that I've seen has cracks that need welded before it's ready for real use again. Usually the engine is about shot if it runs at all. I occasionally see them at farm auctions; most haven't been used in 25+ years and have been stuck in a corner of a machine shed with a layer of dirt and bird poo on them. The reason they weren't used for the last couple of decades is because there are better options for moving dirt. They usually go pretty cheap, but there's a lot of time, effort and money involved in getting them to a usable level of repair.
If you're buying it for a restoration project they can be fun. But if you're buying it to really use there are much better options available than a mini dozer, and for far less than you'll end up spending to make the mini dozer usable.
