Congratulations on your new Pony! Least you have a post “PI” 4.6L Motor, since you moved up a Generation.. (This is a BONE STOCK 04’? A rarity). Seems you’re looking for power, but uncertain. Hope so, as I’m basing this on both of the aforementioned.lol!
Also basing “Broke”, as 500$ or less...
First, you should incorporate a good Tune-up and Oil change to establish a good baseline...
If Stock, breathing improvements are a good target. As a New Plenum Intake, Heads, Cams, HV Pump & Injectors, Long Tube’s, or ultimately- a Supercharger Setup isn’t on the Menu just yet, I’d suggest starting with a Tuner + Cool Air Intake “Combo” on AM’s site (Airaid offers a nice setup- but AM has several CAI types offered with this Combo). Comes with everything you need to Install.
Each also Comes w/the SCT X4 WiFi Tuner (newest) -or the earlier X3 Version (SCT is weeding these out), ensure the Combo includes the ‘lifetime of free Cusomized Tune’s for every specific Mod. you make now, or anytime in the future’ on said Car. You make a Mod, just open the Tuner App., you’ll have the modified Tune in 48 HRs.
You will eventually seek out a Tuner, anyway. So save $$ from buying either item independly, and pick up some respective Performance gains you can feel, it’ll wake your Car right up.
You could also buy a NOS setup, have some fun with it. Stay at/below 50% of Crank (Gross) HP when Gassing it, unless Reciprocating Assembly components are beyond Stock.
Breathing..Exhaust is next.. Should eventually get a nice, Full Stainless Exhaust X-Pipe setup, but that’s a Little more Coin..you DO wanna let em’ know you’re comin’- right?
IF $$ allows- for now- Get yourself a free-flowing Catback Exhaust. Type depends on the type and level of Sound you want to make. Borla, SLP, Flowmaster, etc. ALL have setups that offer those choices. Go to YouTube, people have about every Setup on every Car, there. Listen on in & choose...
Hope that helps! If the $$ Situation is different, Car has Mod’s, leave that info. for a more precise opinion. Cheers!