You would be better off saving your money and buying a used Challenger with a newer version of the 3.5 or 3.6.
The new 3.5's are the exact same engine with the exception of the intake, valve covers and timing belt housing to accommodate a new accessory layout.
The only gain with an LX car is you can get a PCM tuner and tune the settings for well, anything. Where our cars lack that ability, unless you get serious and swap in a LX system or buy a piggy back or independent ignition and fuel system.
Your only "bolt" on options are:
- 3.5 engine with the 2.7 transmission final drive gears
- 98-01 = R/T computer or 02-04 = 300M Special computer
- Cold air intake
- Exhaust modifications
You want to get more serious, there are intake spacers, boring your throttle body and under drive pullies. Removing weight from the car is also an option, air bags, A/c, rear seat, door panels whatever...
You want to get MORE serious, Inherita offers three cams stages for the 3.5... Even polished and polished with big-valve heads. Can also polish your lower intake manifold.
Got $2K+ to drop into it, consider forced air induction routes, or a 4L stroker. High stall torque converter, etc.
These cars with the 3.5, gearings and minor bolt on goodies frankly gobble up most cars people play with as it is. A older gentleman around town has a 300M Special he slow-pokes around and when kids catch him at a light with their Civic's, 350z, Accords, Jetta's, whatever, he blows there doors off.