I have an M1 Carbine and a HiPoint, and have shot the KelTec.
The M1 would have made more sense a few years ago. They have gone way up in cost, both the guns and the ammo. While they have a place in our history and I enjoy mine, they just may not be the ticket for your purpose. Flatter trajectory and better penetration of the 30 carbine over the 9mm doesn't really matter for your purpose, buying bunches on 9mm for carbine and pistol training makes more sense.
The HiPoint is a good shooter, but has a flimsy feel. The new model has a much improved stock. Less flex making it just feel better in the hands. They also have no BS customer service. You have a problem with their gun, they fix it or send you the parts.
The KelTec has the advantage of taking Glock or Baretta magazines, which is nice. For the training purpose though, the ten round HiPoints are fine. The KelTec fold up feature makes it an outstanding truck gun. Nice feature but outside the scope of your purpose.
I'd say pick up a new HiPoint and try it out. You may decide that it's worth getting as many as you need. They are a great value for a knock around gun in a better than 22lr/ less than real rifle round shooter.