1.) Do I need a new FPR? If so, where can I get one?
Ideally, you'll want to use a 1:1 regulator. Most everyone that I know of is using the Aeromotive.
2.) Will the 390s with the 1G resistor pack work using the MSnS?
The MegaSquirt can control those without the resistor pack.
3.) Do I need the wiring harness for them or will they plug in with my stock one?
DSM injectors use a standard Bosch connector, just like our stock injectors. You will need to use the 420a o-rings on the DSM injectors... and they may be tough going it.
4.) I plan on using an in-cabin boost controller to run 4 psi cruising, and turn it up to 8 psi (while the car is running, on-the-fly) for racing/spirited driving. Is this possible with the MS?
That's no problem. You can use an aftermarket manual or electronic boost controller, or you can use the MegaSquirt 'n Spark "extra" code and an additional circuit so the MegaSquirt controls an electronic boost valve.
5.) Do I have to tune the fuel curves every time I change the amount of boost? (would be rather impractical in my view)
No. You tune one VE table. When you're only boosting 4 PSI, it simply never reads the 8 PSI values of the table.
Also, how much should I be paying for the 390s? I have no idea how much to offer on them.
I don't know... but I highly recommend that you use stock DSM blue top injectors that are 450cc. You can get them very cheap and when you up the boost to 15 PSI, you won't need bigger injectors.
I was originally going with just the stock injectors and an FMU/Missing Link but the MegaSquirt looks MUCH better, I just don't understand it yet. I'd like to get some hands-on experience tuning it but unfortunately at the moment I can only picture how it will work in my head, and that makes things a LOT more difficult for me to understand -- I'm a hands-on learner.
Keep reading and don't rush into it. Take your time and learn everything you can from www.megasquirt.info and www.msefi.com
You can also download the MegaTune Software to familiarize yourself with the tuning interface.
Good luck! If you have any more questions, ask away.