I'd grab the two 280zxs, but I'd offer between $350 to $400, since he wants both gone, if not offer the $250 for the cancerous one.
Or you could be a real dink about it and give him $250 and say that's all that you are giving, but "since you want both gone...".
Anyway, from what you've described, a running 150HP car is better than a non-running car. So the 280ZX swap would be on my list right now.
I have an L28 in my 240, that I also turbocharged and am having a LOT of fun with, other than one of my 90* couplers splitting yesterday.
I used an N/A dizzy in my conversion, but I'm also using a GM ECM to run the works, so my needs are a bit different than most people's. Mega Squirt can be configured to use the N/A or turbo dizzy.