Don't know how much "value appreciation potential" a 930 cab has. There were certainly a lot made (and a fair number of grey market ones, too, diluting the demand:supply ratio). You're certainly not looking at the next numbers matching Hemi 'Cuda phenomenon, here.
On the other hand, I don't think they'll depreciate too much farther. Looking at the value guides periodically published by Bruce Anderson in Excellence, 930 values seem to have been fairly flat the last few years. I'd have to go home and look them up to give you specifics, if you want. I figured worst-case-scenario: early 930's (from the late '70s) go for the low 20s. So that's the absolute lowest I'd ever see later 930's valued at, before they'd maybe slowly climb up again to be labeled as "collectible"--roughly the same time we run out of gasoline anyway.
If you were willing to sacrifice some power (how fast are you going to be going in a convertible, anyway?), you could also look into a normally-aspirated 911 cab (with or without the Turbolook option). Simpler to work on. More engine longevity. Cheaper to buy. And not something you see everyday, anymore.
1987 Venetian Blue (really looks like grey) 930 Coupe