Does your existing grinder do espresso? if not you may need to also factor that in.
Another factor is whether you're okay with a manual machine that can't froth milk. (E.g., Flair, Uniterra Nomad, Wacaco Picopresso, Cafelat Robot). If you're just doing straight espresso or you don't mind picking up a separate milk frothing device like a nonfoamer, this can save you quite a bit over a machine with a boiler and pump. However, the workflow will probably be more fuss.
For a full machine sub $500 I think a lot of folks go for something like the Breville Bambino or the Gaggia Classic. Some modding may be required for best results on the Classic.
(My espresso setup is an 1zpresso J-Max and Uniterra Nomad)