Hello,
If you live in Canada, I doubt the pond is a good idea. Gets a little cold up there, no?
40 is a little small for most mbuna, I think your best bet is to find a species you like which is on the small side, and do a single group. Pseudotropheus Demasoni would probably be the best choice. You should probably do no less than 12 and no more than 18 or 20. They only get to about 3". The key is to get mostly females, the males will hate each other. But with enough rockwork and females in the way, they will be able to hide or escape.
As far as the filter, I have always liked aquaclear. You definitely want to oversize the filter since you have to pack mbuna tanks more tightly than most fish. An aquaclear 70 or 110 would do the job, I wouldn't go smaller than 70. 110 would be better.