Great thread about the 4kT. When I bought mine back in 2000 it seemed rather complete from the outside, unfortunately it wasn't. The important bits were there, the parts that made it an A4000T but there were no drive rails and some other internal hardware was missing as well.
However, sure we all want mint condition A4kTs but chances are that you'd only find a good functioning but less than complete A4kT. The good thing about the A4kT is that it fits inside a bog standard AT tower case: no hardware modification of the case should be needed to put the lady in! Okay, AT cases are getting pretty rare but they're easier to obtain than mint A4kT cases.
The conclusion is that you should not leave the system at the seller's just because the case is incomplete. If it is working problem free, the case should be less of a concern.