Sorry to have put a bit of a dampener on it, but at a budget limit of $500 you are just not going to get a decent grinder with doser, unless you are happy to go with used ex-cafe grinders, etc. Owners of Mazzer Minis even report issues with the dosers on these, although Macap M4/M5 grinders apparently have very well designed dosers fitted to them.
The dosers fitted to the likes of Rocky grinders, etc are pretty flimsily built and not generally considered to be very well made, more of an after-thought than anything else although some owners may be prepared to defend them. If I was you mate, Id either revise the budget to something like $700+ or consider the 2nd-Hand grinder market. All the best,