diablosport, cold air intake and exhaust is very nice for the beginning.... my best recommendation to it is a set of pulleys. they work really well on my car.
they sell cold air intake, diablosport, exhaust and pulleys as a package, so you save money when buying a package.... u save around $150 in total.
later you can also add a nice set of 4 ;10 gears, short throw shifter and an electric water pump or something.
and try new subframe connectors from either granatelli motorsports or BBK (BBK $ 90 dollars more and does not differ much. they improve your steering and eliminate body roll in both - drag and track. i love them on my car. it's an amazing modification.
if not enough power SUPERCHARGE IT. hahaha u can make 340 rwhp (around 395 on the motor) when supercharging a mustang 4.0 v6.
and the supercharged v6 mustang
will beat the **** out of the stock v8 mustang on the straight line, on the track and on the fuel mileage.