Have you checked the car sales on this forum and
https://m3list.com/? Even facebook marketplace can have a gem out there. You may have to travel to get one but there are plenty out there. Also may have to settle on a higher mileage one and have money left over for mods/maintenance. I just snagged a 2011 E90 M3 DCT Base with 124k miles for $21.5k with a solid maintenance history (needs RB but TA done) and one owner, wasn't looking for a competition as I am likely going to change the wheels and suspension on it anyway so that can be an option for you as well if you were ruling the base model out of your search. Good luck!