My current slug gun is a Mossberg 935 toting the camo slug barrel with the cantilever mount. The barrel was $175 from Buds Gun Shop, online. I haven't seen any USH for that price unless they were used. It's very accurate with Remmy Buckhammers and Lightfield Hybrid-EXP's out to 100 yards. Off the bench, they group very tightly. I haven't really seen the need to go any further than that.
The reason all Mossberg 835 and 935's say "no slugs" is because the typical barrels that come with these two models are overbored. It has nothing to do with the chokes. The bird barrels on these models are basically at 10 gauge specs and would be very inaccurate, not to mention potentially hazardous, with regular 12 ga single projectile ammunition. You can, however, buy a slug barrel for either model, be it 835, 935, or even the new 930. To my knowledge, the 535 is not overbored. They have a variety of slug barrels available.....it's just a matter of picking the one that you want. My advice would be, if you're using a scope....buy the cantilever mount. It's going to be much more accurate than a scope mounted to the receiver.
Edit: I just did a search for the camo, cantilever mount 535 slug barrel (mossberg accessory barrel # 91315) and found many for around $160. A brand new USH will cost you around $220. (in my neck of the woods.) FWIW, any Mossberg slug barrel that I have ever shot has been accurate, and I can name about 4 more guys that I know that hunt with either a 500 or 835.