Thanks to all for the thoughts and input. I'll contact the raft repair place to see what used boats they have for sale. I just don't want to spend $800 on a used boat, $250+ on fancy full size oars, etc and then have trouble selling this stuff near DC if I don't get serious about the hobby.
I know the Mariner setup is not ideal, and I might waste $200 if the boat itself sucks/fails, but the oars were $30 used and the frame components could be reused on a better raft if I upgrade. And since I just want to do fishing floats on relatively calm rivers, I'm going to try it out.
FlyingDutchman, Kanzam, and Cupido, thanks for sharing your experience and recommendations! I thought I was the first to try putting a row frame on these little Mariners. Folks have tried them paddling them in whitewater before though...check out this vid at 1:50:
(it is the Seahawk II, an older model of the Mariner boat, but with inflatable floor, which they converted to self bailing).
Cupido, did you use 1" cam straps through the molded plastic loops? I'm planning to lash the frame to at least 4 of those smaller rope guide loops plus the molded plastic oar mount fitting (possibly putting a bolt through the fitting where the oar lock used to be, as a more durable tie down point.).
Do you think pipe insulation on the underside of the frame would work as a wear-guard? I'm just thinking that inflatable kayak pvc + glue will cost another $50 on a $200 boat. Maybe it's worth doing it right on that, though.
The bilge pump idea is brilliant. My boat has the inflatable keel too, so I'll borrow that idea in case we take on water.
Did you guys run class 2 with the hard floor in? The floor fits in there pretty tight and I'm a little paranoid I'll hit a rock that will send one of the hard floor slats right through a side tube (POP).
Thanks again for the great food for thought. I'll post more pics/reports as my boat comes together.