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+1 for the Stag!
E46 bmw 330i
Bringing this forward because in 4 pages I still haven't seen anything better. For 10k you can get one than won't be a mechanical nightmare. It's fun to drive, has power and huge aftermarket. They are increasing in value. It won't scare your clients.
If you want older a clean E36 works too.
Consider a late 90’s to early 2000’s R129. Can be had for a decent price. After that, move up to my realm.

Still undefeated for cowl shake.
They vary a surprising amount, I’ve been in some where its absolute awful and others where there is next to none. I have no idea what makes some good and others bad though, it seems almost random.