first congrats on your sucess. it is amazing now days the opportunities people have at any age to become a sucess.
i do agree that you are very young myself included (18). you are 21.5 on the way to sucess.
335i is a great car which gives you both luxury, practicality, and power, and it fits your age. if you wanted to move out of your parent's house, definatley keep your 335i and invest in the house as that is one of if not the best investment you will make in your life.
but if you plan ahead and believe you will still have enough money in two years to get your appartment then you all good. its all about planning ahead, organization, and management.
as far as the m5 goes, it is an AMAZING CAR. more car then anybody needs.
v10 500HP and those things take off like nothing else. yes it is alot gas deficcient then your 335i and yes you have to either a) use the paddles or b) use the lever to shift not unless you get the manual. if you want more info on the smg you can look here, http://m3smg.com/
. i used to have an m3 manual but know alot about the smg. its pretty simple.
in general you got 6 settings which are shown on the bars increasing. the higher the bar the faster the shifts go. so on normal driving you would set it on the one for you most people got for s3-5.
note that you have a "m" button on the m5 as well. so at a touch of a button your "race" presets will activate. press it again and it goes back to your casual settings.
the suspension got modes, you get the m, optional usage of 400hp/500hp. if you need anything else let me know man. i can keep going all day long