Looking at a 1975 300d with "low compression"

[sailorbenjamin]

I haven't seen it yet in person but it looks really good in the pictures. Just been in storage seven years and won't start. I like the OM 617 in the W115 body, and it's red.
I have two questions;
First, the car doesn't look like it's got enough miles on it to need a ring job. I think the problem lies elsewhere. Maybe the fuel pump isn't holding up to the new fuel? Any ideas?
Second, assuming the worst, and it's really dead, what would I look for in a replacement engine? Bolt in compatability is nice, but I might be willing to do a little more work for a turbo or something. I guess I just want it to fit the tranny and the engine mounts and not get to radical. Anything I should specifically avoid? Would a 6 fit?