415HP from 318ci IS POSSABLE!!

Location of where one lives can have a cost break. But then again, the pay check may be different as well.

$2500 isn't to much to build apon and I would suggest a bigger bank roll for this. Better off waiting rather than trying with less money and possible cost over runs.

I'm more so with Moper on the piston selection and would rather go that route and zero deck them.

Last comment;

415 or even more HP from a teen is fine to go for and actually do and then brag about. Go for it if you want to do it. Just don't get hung up on HP numbers because it comes at a loss of torque expense witch is actually a better asset to have.

Big HP numbers reduce drivabilty and to get it back requires steeper gears and higher stall converters which are a draw back for a street car and reliabilty/longevity issues.

Have I been there? Yes.
Was it fun? Yes.
Will I do it again? Yes
Do I expect it to live long and travel the country? Hell no. But a terror around town for a season or two is all it'll do and do it just fine.