if the engine sounds like a wheat thresher, the bottom end is wasted and will need bearings. make sure the frame isn't trashed or cut up. $2500 is about the low end for a running one (up to $4000 for the nicest one out there). Unless it is a project then it is the high end for a project.
an Ironhead is the colloquial nickname for a sportster motor that has...drumroll....iron heads (or just the rocker boxes I can never remember). They are also called shovelhead sportsters because they look like shovelhead big twins with sportster bottom ends. They were made from 1957 till about 1985
A sportster is different from a big twin in that the sportster engine and tranny are unitized like every other motorcycle you have ever ridden. A big twin has the trans seperate. drive belt to the rear wheel on a sporty is on the right, on a big twin on the left.
there is so much info out there, just type history of HD engines into google and you will get a million hits. When in doubt start with wikipedia.