Are you going to do the work yourself? Are you comfortable with cutting/welding, etc? Or do you want to be able to swap on a short barrel?
Those are really the important questions. If you want swappable parts, you probably need a 500 or 870. I went with an H&R Pardner Pump 12 gauge, myself. It was a little less than $200, and is a clone of the Remington 870. The only aftermarket parts that don't fit are barrels. (I'm not sure what the difference is, but there's apparently some difference.) That didn't matter, as I was chopping the barrel myself.
I chopped the barrel down to 12 inches, and shortened the mag tube so the cap is about 1/8 to 1/4" shorter than the end of the barrel. It took a little work to do that. The barrel chop is the easy part, really. After lopping off the end of the barrel, I removed the magazine tube lug (band? not sure what you can it, but the piece that is attached to the barrel and connects it to the magazine tube) from the piece of barrel that was removed, by grinding it off. Then I chopped the magazine tube down as well. However, the end of the magazine tube is threaded for the cap, so I actually cut off those threads as well, and then tack welded the threaded section back on to the end of the mag tube. Then I ran a dremel sanding wheel inside and out to smooth out the welds. At that point, I slipped the lug over the tube, screwed the cap on, and then tack welded the lug back to the barrel. (I would normally worry about welding on a barrel, but the lug is literally within 1/2" of the end of the barrel, so I don't have any strength concerns.)
Once everything was welded in place and smoothed out, I used cold blue to refinish it. Then I drilled and tapped the end of the barrel for one of those little fiber optic front beads. I also replaced the furniture with Magpul's set for the 870, adding a short rail to the front with a Magpul hand stop, since the foreend is so close to the end of the barrel. I used the FDE furniture, and dyed it with dark blue RIT dye.
Including the tax stamp, I have less than $600 into this project. (Less than $200 for the shotgun, $200 tax stamp and less than $200 for the Magpul furniture, bead sight, and miscellaneous supplies.)
I'm not going to tell you that it's the prettiest SBS in the world. But the welds on the magazine tube (for reattaching the threaded section) are covered by the barrel lug, and I cleaned up the barrel lug welds pretty good. The "worst" part is probably the cold bluing and subsequent wear on it. But it's a solid shotgun, extremely handy in this size, and you still get 4 2-3/4" shells in the tube. With some low recoil buck, it's a primary HD gun at my house. And anyway, the main reason I built it was because I needed a Form 1 project so that I could beta-test eForms and write a walk-through for my clients.
I'll post some pictures if you're interested.
Aaron