It's about in the same state as the one I had before the previous owner restored it. Mine did have running gear though. After he stopped counting at £6600 of bills, some of which were self inflicted, restoring it, the same as any car, from that condition isn't going to be cheap. Also, think about every join, weld, panel condition, seams, sealant, rubbers and rustproofing and inherent bad design (rear light clusters for example). Just because you've done a panel once, doesn't mean in a few years you won't have to do it again. SD1s can be rustproofed, don't believe what people say, but you have to think and plan and use very good quality materials.
As for an engine, remember that the 3.5 is the most robust, then the 4.2, 3.9 and lastly the 4.6. depending on what you want to do, you can get more than adequate power out of a 3.5 if you talk to the V8 tuning guys. Lastly, join the SD1 club and be confident that you're doing the right thing, restoring what is a pretty rare car now.