MODERATOR NOTE:
Very sorry for all the snotty answers. You asked a simple question, which I outlined in bold. I think part of the issue is that "where to look" is ambiguous.
Are you asking what to mod on the car or are you asking where to go/what to buy to mod the car?

For what to mod:
Smaller supercharger pulley
A REAL cold air intake, unlike what the factory put on the car.
Upgraded injectors, throttle body, fuel pump.
......... do all the above and then get a tune which will take advantage of the above mods.
The above will yield an impressive car (and there are other minor upgrades as well) but the above will set you back no less than $4000.

For where to go:
Needswings.com
https://themercedesswapshop.com/
....... there are others but those are the guys I know and would personally trust.

Now, there IS truth to the fact that you can buy a modded car that will KILL for way less money than doing the mods yourself. That is, the value of your modded SRT is less than what it cost to buy it and mod it. I THINK that was the reason a few were essentially not answering your question and instead offering the advice, "Just buy this car...."

I really wish those who answered, if they are not going to answer your question, would have taken the time to explain WHY they are not answering your question and are opting instead to just tell you to "buy a car already modded".

MODERATORE NOTE:
I am going to leave the thread open, but if the contentious tone continues the thread will be closed and locked.