I enjoy building my own frames (see the 'Games with Frames' thread posted last year for sources and methods) with steel tubing, Hollaender SpeedRail joints, and various custom fiddles.
This is the frame I built for my Jack's Cutthroat tubes, set up as a support boat, with a separate gear frame front (holds 2 York Packs) and a rudimentary rear seat in case someone needs a ride.
One thing about a modular frame is that you probably won't
sell it. Instead, you can keep the joints, oarstands, etc. and buy different lengths of tubing to fit whatever new boat you've picked. You can also change the dimensions according to the requirements of a certain trip.
I just bought more tubing to lengthen my big-boat frame for a different load configuration. Total cost for 28 ft. of top-rail: $42.27. The same length of NRS aluminum tubing costs $112 + shipping. But that's still a lot smaller bite than a whole new frame. I've got a tubing cutter, a vise, files, etc. But the tools are likewise, cheap.
Whatever you do, good luck.