Welcome and congrats on your purchase.
IMHO its not worth spending money on performance upgrades on a 320i. Its diffifult to squeeze power out of that engine, especially since you say you on a budget.
Spend the money on exterior accessories and make it look real nice. Lots of guys on the forum that can help you with that and rather keep the engine standard and do a nice exhaust to make it sound nicer...
just my opinion...i owned a 320i before.