That Mossberg is the same gun as the Tristar Viper and Weatherby SA-08, just not as finely finished out. A stone and some oil and it will run like a Swiss watch.
28ga #8s and #9s are plenty in 410 or 28ga, 5/8oz #9 load cost about $1.68 to load at today's prices, new primed hulls, powder, wads...