I shot open with a saiga last year. I took a couple of months to work the bugs out but in the end it was very reliable. I did all of the work myself and have $1500 in it.
The main issues with a saiga or any mag fed shotgun is dealing with multiple loads on a stage and solving a miss. Example would be 7 rounds shot - 2 50 yds slugs - 4 shot - 2 75 yd slug all from different positions.
You can buy insurance by loading extra rounds to compensate for a miss and burning rounds if you don't or doing multiple mag changes. Once your mags are loaded you are stuck with your decision. When everything works out it's great when it goes bad it's brutal.
With a tube gun you can single load for anything that comes up at anytime pretty quick.
Most of the club matches I have shot it would not have been a disadvantage to just have large extention tube that would let you start 12 or 14.
So to answer your question for a cheap open shotgun I would buy a 28" mossberg 930 waterfoul and add a nordic +8 extention and maybe add a red dot if you have extra cash. You could add a speed loader or xrail at a later time if you decide you like open.
Knowing what I do now I would have went with a xrail.