Replacing aftermarket HU with stock

[screwcancerletsrace]

I have a 2008 BMW 135i and it has a garbage Kenwood head unit that I'd like to replace with stock. Before I purchase one, I have a couple of questions.

1. Do I need a specific model of head unit?

2. How do I find out what model I need?

3. Can I just put one in from another car without issue?

4. If no to 3, what do I need to do to make one from another car work?

Thank you.