I'd recommend trying a spring shotgun first to see if you like it before getting a marui. They are pretty easy to sell after you're done with it and you'll have all the shotgun shells you need since both types use the same.
I have a cheapo springer and it works great, love it as a backup. Cheaper to get "mags" then a pistol, and surprisingly good range for a spring shotgun.
That said, if you don't have an AEG get one of those first. Shotguns are fun, but you're out gunned no matter what and the advantages of a shotgun don't really translate to airsoft.
Cyma's got some new shotguns I've been seeing online for sub $200 that look a bit better then the ones most people have.