I wouldn't say there's any hard and fast rules about starting on 4 vs 5, there's a bit more muting technique necessary on a 5, but learning it early isn't a bad thing. Music is music, there's no difference going from 4s to 5s or vice versa in my opinion, if you have good technique and an understanding of the instrument, you'll nail it in 5 mins either way.
Of course, if you don't know why u want a five string...why pay the extra money for one?, it's not really the case that it's more versatile, otherwise we'd all be playing 8 strings!. Usually it's building from the ground up, getting to know your preference, which is why most start on 4's.
Buy second hand but do your research, get recommendations etc
Si