100 K miles, huh?
If you trade it in, it isn't worth anything anyway.
If you buy a MSP, you will de-value it with any upgrades.
You don't think that you will be able to go faster in an upgraded MSP versus a turbo MP3 or P5 do you? You will need to do just as many mods.
I am not really sure what you want. The MSP is not a magic car. If I were to go fast and REALLY mod, I would rather start out with an MP3 or P5.
Here are your options, the first 2 are smart -
1. Keep the MP3 and get a sleeper kit, saving for a built engine and junkyard low mileage tranny (and engine management and LSD - which I really recommend).
2. Get the MSP and buy upgrades to it (you can get a manifold, downpipe, etc. from MAM).
3. Get the MSP and get a built engine and thumper (you would get the same performance in an MP3 if you did the same, just wasting money on the MSP, itself, if you do this).