Ditty Wa — Interesting project you've set yourself. I play a lot of blues on guitar and old-timey, banjo and fiddle music on Autoharp, but hadn't tried blues on the harp.
Listen to Don M. 21-bar harps suck. They use foam rubber rather than springs for the return, the bars take up too much space and they're generally harder to modify.
A 12- or 15-bar harp is what you would need for blues. Some serious players even have a whole bunch of 5- or 7-bar harps. A lot depends on how much money you want to lay out. I suggest a good 15-bar to start with. That'll give you an idea of whether you want to dig deeper.
I would look for an Oscar Schmidt birchtop (one step down from the Appalachian) and the main things to look for are cracks, top separation from the body and loose tuning pegs. All are pretty rare, but can seriously inhibit playability.
The great thing about Autoharps is that you can cut pads for any set of chords without much trouble. There are hundreds of instruction manuals out there, but the things are pretty simple to figure out on your own.
One trick I learned to keep the bars from clacking when you're playing quickly is to get some of those little green felt pads that you put on the bottoms of vases, cut them in half and (after removing the two screws each and the bar covers) put one on each end of the bar. You'll never hear a sound out them, just pure harp.
Also, if you do get the birchtop, you may want to unscrew the bar assembly from the instrument body, measure about an inch toward the bottom (using the line of the original holes), redrill and move the bars down. That will give you even more room to pick between the bars and the tuning pegs.