1st off welcome brother.....
2nd - The cars are excellent for not doing much maitenence on them. Most BMWs when taken care of well will last over 200,000 miles.. But I am assuming yours has a lot less..
3rd - Oil changes can be done in accordance to the meter that is on the instrument cluster, but is recommended you use synthetic oil like Mobil 1 or Royal Purple which I use and change the oil ever 6,000 miles.. while you will want to replace your OIL filter every 3,000. Other maintenence is the same as any other car.
4th - 18" is going to be the biggest you are going to be able to go to without doing fender lining mods, rolling fenders, or spacers... although depending on the rim width, and offset, you may still need wheel spacers between the wheel and hub.... Plus if you plan to alter the suspension down the road, i.e. lowering springs/coilovers, you will need to have your fenders rolled and some clearance checks done with 18". 19" fit nicely on lowered 3series cars but require all of the alterations in order for them to work nicely.
Basically if there is anything else you need, feel free to ask..
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