First of all, welcome to E90post
I would get one of the newer models because the late 05, early 06 models are normally more prone to problems since they were the first e90s to be made and minor imperfections that were later found out were fixed in later models.
If you do go for an 06, there are 2 choices. 325i or 330i. I would go with the 330i without question if you have the means. 40 more hp really does make a difference. If I were you, I would go for a newer car with lower mileage. If you have the means, I would recommend an 08 328i.
I have an 07 myself with 87k kms (around 55k miles). I can tell you that our NA engines are pretty bulletproof. I have experienced issues with the car however. Things you should look for in general are (What I have experienced) :
1. Door lock actuators, mine failed at 76k kms.
2. Valve Cover Gaskets, Filter Housing : Mine had a small leak at 83k kms. Pretty $$$ fix
3. Starter: Failed at 87k kms for me. It is rare for an e90 starter to fail at such a low mileage but it can happen. A costly replacement as well.
4. Spark plugs : Mine are fine but are recommended to be changed at 100k kms.
5. Battery Replacement : You will need to replace the battery if you are buying a 06/07 car indefinitely if it has not been replaced before.
Keep in mind that many of these problems will not be able to be detected during a Pre-purchase inspection. Most importantly is for you to test drive the vehicle, if you feel anything wrong with it, do not buy it.
Good Luck on your purchase!