First and foremost, attend a match or two and see what everyone else is using to get an idea of what you might like, or what works/doesn't work.
A shotgun with detachable magazine will put you in open. (assuming the clubs you will be shooting at have divisions etc)
If you want to compete in open, then a mag fed pump might be the way to go, but I have not seen a reliable one. Constant feeding problems. I'm sure there are examples of reliable ones out there, just haven't seen one.
Had to look up what the DA Cougar looked like. I personally don't like the Remington style bolt release button being incorporated into the lifter, and I don't like the fact that the fore end covers part of the loading port.
These will make for slower reloads vs other platforms.
All depends how serious you want to get into 3 gun or practical shotgun.
If you can spend a little more, look for a used or new Mossberg 930. If you can spend a little more than that, people in the US have been having luck with the Remington Versamax, and ultimately I would recommend a Benelli M2.
For a pump, you can probably find a used older Wingmaster on the cheap, add an extended magazine and be good, but a Mossberg 590 or Nova/Supernova are great choices as well.
You have to decide how committed you want to be to the sport, and buy accordingly. You can continually upgrade or just buy the best and focus on competing.
Buy once, cry once.