Hey, a little late to the party but wanted to chime in.
Nathan (MySteed) hasn't done kits in YEARS. Don't count on that option. That price is from back when the M112 was first becoming an option. That price was (at the time) about a $1000 less than any other complete supercharger kit for the 3.8L V6 (Vortech and Pro Charger being the only two others). That being said, I'm not sure Andy (TMA Turbo) is making V6 kits anymore either.
If you pay attention to the various forums/FB V6 groups, you can find one of my old kits floating around. They go anywhere from $1500-2000. Ortiz Performance and VMP both can remote tune it for you. Avoid a knock off - they don't work and will more than likely cost you a bottom end from bad air flow. You can also find old TMA kits floating around for about $3000, if you really want a turbo. I've seen intercooled Pro Charger kits go for about $2500. I don't keep up on used Vortech prices.
As for the 3.8L vs 4.2L - unless you are planning a full build, stick to your 3.8L. Both stock bottom ends max out at the same power level...and both will get there with just boost and fuel on an otherwise stock motor.