SPAS
No, SPAS-12's aren't destructive devices and they are perfectly legal to own (unless your state has some sort of prohibition against them). You'll find pre-ban models with the top fold stock and post ban models with a fixed stock and short mag tube.
A quick look on Gunsamerica shows an average price of about $1200 now.
I had a SPAS-12 for a few years and was umimpressed with it. It was very cool and intimidating looking but a less than satisfactory performer IMO. It was picky in semi-auto and kinda clunky to use as a pump. It was a pig, big heavy and awkward. I sold mine off and bought a Remington 870 that I was a lot happpier with. It's not semi-auto but it's lighter, simpler and works well.
The SPAS is an interesting collectable but for a serious fighting shotgun I'd go with any of the American pumps (870, Mossberg 500, Winchester Defender) or a Remington, Benelli or Beretta semi. Especially for the money. You could get 2 1187's or 1201FP's for the money the SPAS's are going for, or 3 or 4 good pump guns. If you really have to have the pump/semi option then teh benelli M-3 is a much better way to go IMO.