It is almost certainly a model 500. I do not know why you think it is too good to be true, that is a typical sale price for a common combo from a big retailer who can buy in bulk, especially on a big sale weekend. The short barrel will be cylinder bore, in keeping with typical security barrels. Mine from twenty years ago had a fixed modified choke on the long barrel, and that is what is probably on this one.
You can tell by looking at the muzzle, if it looks like it has a sleeve inside with some notches, or has some knurling on the end, it is a removable choke. My feeling is that if you do not know how to tell, I doubt it would make much of a difference, especially when its that cheap. I have guns with interchangeable chokes, and am too lazy to ever change them. The fixed modified is usable for most stuff. The only time it should matter is if you waterfoul hunt, where you want tighter chokes, in which case, you do not want a standard 12 gauge anyway.
The Remington that Big5 also has on sale is nice, but may not suit the original posters reason. If he wants to bird hunt, and have a defense/fun barrel the Mossberg combo is a good way to go, otherwise, if he gets the $300+ Remington, he would have to spend another $100 or so to get another barrel for hunting. The Mossberg is fine for a light-medium duty gun, and as a southpaw, I like the safety better, and the loading gate is easier to deal with. Though they have has a lot of production runs, and some have been better than others, I would handle the gun first before buying it.
