If money was not an option:
On my E46 318i.. I would turn it into a sleeper.. Keeping it looking sort of stock..
Would love to build a high revving (9500rpm) 4 cyl something like a S42B20 but high revving and turbo/Super charged together with a 6 Speed box & a LSD at the back

The main reason for this, is everyone always says "get rid of the 4cyl for a 6cyl" so to prove them wrong I would stick with a 4cyl...
Would upgrade all suspension bushes
Fully adjustable coil over system
Brakes obviously would need an upgrade which would need a wheel upgrade to a set of 18" E90 Msports
looks wise, I will keep it stock, Besides the Wheels, Would put the 318i badge back on
Interior would need a set of BMW sport seats & steering wheel
Think that's about it..