Is it a John Deere 622 or a 2010? And where can I find out about it?

I found a dozer advertised as a John Deere 2010. I looked at it today And the ID plate says John Deere model 622 with a ser# of 702 or 792. Since i'm a newbie i dont know. Is there a place i can research this model and ser#? The dozer looks pretty good but the paint job is not that old. One of the large plate wheels has the date 121862 stamped on it. Maybe year of manuf? Unkn hrs on this dozer. It has a hydralic hose leak and the seller is asking $8950. Should i resaerch some more or run away quickly? Thanks for any help anyone can give me.