Hi and welcome to the forums. I'll answer your questions one by one.
1) Yes that is reasonable.
2) 320i is a fine alternative if you can handle the lower hp/tq. It is not a "slow" car. You may have to search harder for a 320i with nice options vs. 328i which comes with some standard.
3) I haven't been in the F3x game for long so I can't really say. I have seen some people have great experiences and others not so much. You're already ahead of the game by looking for a 2016 or newer as they do not seem to suffer from the timing chain issues.
4) When you say "great" what do you mean. It's all about what you prefer, it varies from person to person.
If you aren't set on having a sedan, have a look at the 4 series they come as a two and four door "coupe", and come standard with some niceties like Xenon headlamps whereas many 320 and 328 come with old fashioned halogen headlights.
2001 Z3 3.0i -Oxford Green/Sandbeige
2016 428xi -Estoril Blue II/Black
2018 430iC- Estoril Blue II/Black
2018 330it - Melbourne Red/Venetian Beige/Black
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