Indeed. Someone recently posted in the Vintage section a link to a Bring a Trailer auction for a W115 240D that went for $5000. The car was a repaint with non-original cloth upholstery on the seats.

http://www.peachparts.com/shopforum/vintage-mercedes/382764-bring-trailer-featured-w115-240d-manual-auction.html

Perhaps W115 values are on the rise as they become more rare and people start to appreciate their place in M-B history. It also seems there is a fair amount of interest in the chassis amongst younger folks looking for an older M-B who want something besides a W123.

If the car is nice and you like it it's probably worth the rebuild, especially if you're looking long term. You've already done the body work, sounds like. How's the interior? Suspension? I'd be attached to it after all the effort invested so far.

As for re-using your recently replaced parts (timing chain, water pump, IP) - Whoever builds your engine might not want to offer a warranty because of the "used" parts. You'd just have to communicate and come to some kind of agreement with the builder.

And FWIW I'd put in a new oil pump.