Mini 14's are fun little guns to shoot,but the price has gone through the roof for what you get. If they aren't rapid fired,they will keep decent accuracy.
I've never had or shot a Keltec SU-16,but do have a PLR-16 pistol and it is a blast too shoot,and pretty accurate.
The Saiga is probably the best deal for the money.It's accurate,will shoot almost any ammo,and will enjoy getting as dirty as you let it between cleanings. The only downside to the Saiga is mounting a scope,the way the rifle is designed a scope sets pretty high over the receiver and takes some getting use to.