630 Csi

I am looking @ purchasing 77' 630 CSI. It has 48,000 miles and the exterior is in very good shapefor 30yr old car. The cyclinder head has been replaced on the car and some other engine work done. It was driven to garage a couple of years ago an has sat ever since. The owner is asking $4,000. I think it is a fair price but sure not how it is going to cost to maintain or its reliability. Any advice or comments?