closest I can think of is erpnext or odoo

This is wildly unwise. Unless you're willing to be on the hook for when things go wrong (either technically, administratively, compliancy or financially) and be the tech support on the most inconvenient of times, just pay a service to do this for you.

This is one of those 'cry once, buy once' situations. Not worth the risk, especially when it's for a main source of income.

If in the United States, new federal laws were recently passed that govern how car dealerships handle client/customer info. I’d advise everyone in that space to familiarize yourself with them.

A "bloody beginner" should not be managing something of this nature.

Hey, we have built something very recently (about a month back) which has everythign you need except 2nd point.

Can resell the solution, lmk if you'd like a demo!