Wow that is a pretty brutally honest seller. I think if he is telling you the battery would need to be replaced, I would not go near it personally. And 'control module' could be a very serious fix too. And those are just known issues - what about things he hasn't done that aren't broken yet, like the RDM fix.
When it comes to cars in an 'inoperative state', I think you have to look at auction listings which usually have body damage also, so hard to say, but if you can get a new one for mid-30s that has NOTHING wrong with it, then 20 seems pretty fair for something in this condition.
Karma #328: Shadow on Monsoon Tri-tone