It's really more about what you should avoid than what you should get. Avoid:
-turbos
-all wheel drive
-V8s
-automatic transmissions
-direct injection
-iDrive
-air suspension
-convertibles
A non M e36 is a stinking good time, easy/cheap to maintain, and you can get a really nice one for $4000.
An e39 530i 5mt is in budget and a better driving experience than anything BMW makes today.
You should never go to the dealer with whatever can you get (or any car post warranty), and you'll have a better experience if you enjoy working on your own cars (which is far easier on everything pre e9X than post e9X).
After ~100,000 miles, mileage becomes somewhat irrelevant-- becomes more dependent on when the last time things were replaced. e.g. most bushings are worn out at ~100,000 miles. If they're still worn out at 150,000 miles, you're not really any worse off.
... you might find you like your F30 less after you get used to the dynamics of an older BMW
