I've not looked into P2M at all to be honest. I'll need to get new cranks as well as the unit though right.... And do any brand chainrings work with them?
The cranks depend on whether you have a pair of compatible cranks already. I did, so just ordered the P2M to fit my cranks. If you don't have compatible cranks then you can order the cranks with the P2M already fitted.
The chainrings aren't an issue, use what you have or get new ones.
With that budget you can get a P2M and some change.