I have never seen a Natal Plum growing as a tree, and the tallest one I have ever seen was my Grandfather's, which was about head-high when it was 20 years old, growing in the desert with lots of heat! Most are about hip-high after a few years, growing in zone 9.
I don't know if you could grow one very well in zone 5, and I doubt you would get fruit. You do know they have leaves and need sun all year long, yes? And they do not do well with cold at all. Even I have difficulty growing mine in Sacramento, because they keep getting nipped/killed by frost when our weather reports are wrong, and I don't get them covered. I have replaced 3 plants that died this way (well, 2 and one that looks REALLY bad right now :( we'll see if it grows again when it gets warmer)
But if you still want to try, I would love to know where you get seeds, because Natal Plums are one of my favorite fruit, and it's getting harder all the time to find anything other than the ornamental "Boxwood" variety.

Carla in Sac