Here are my thoughts on modding the 4.0L. I was one of the first batch to start modding mine back in 2005 and had a ProCharger P1SC kit ordered the day it came out. I eventually bumped it to a D1SC (SO much better), then did Super Six Motorsports heads and cams (Tom is still around and in business, and is your BEST source of info). Total power with heads and cams @ 14psi was 440 RWHP. Good for a V6? Sure, but there are some caveats here.
Anything over 400 RWHP is pushing it. The only reason I ran that much boost for as long as I did was because I lived at 6500ft (Colorado Springs). While we only have 91 octane here, the altitude makes detonation less of a worry. Unless you plan on using aftermarket/forged rods (which weren't available when I built, but are now), that kind of power leaves little room for error. With that, though... you're also talking about forged pistons, and a seriously PITA engine build. This is really just not worth the hassle, IMHO. Get a 4.6L 3v. There's no way they are 8-10k. Whoever's giving you that price is smoking crack. A quick search at autowrecking.com (first result in my search) showed $1600. You'll also need a PCM ($200-500), the GT engine wiring harness ($200 from Ford, but that was years ago), a transmission ($1000-1500), and a few other odds and ends. This may sound like a lot, but you are going to spending a LOT more than that on building a 4.0L up.
The other thing I cannot understate is the 4.0L is just a shitty sounding engine with "good" cams. You can look at youtube.com/cabuzzi for some of my old 4.0L videos. Outside of the first heads and cams video (which was with open headers), they all sound like shit/rice. I made revision after revision to my exhaust, trying to find something tolerable. I eventually said fuck it and decided to do a 3v 4.6L swap (don't ask me why I chose the 5.4L 3v, though...). The shitty sound of the 4.0L was the #1 reason I swapped to a V8.
I would recommend one of two courses:
#1 - Keep the 4.0L stock and get a decent power adder, but keep it no more than 10-12psi. That should net you 300-350 RWHP. You'll be happy.
#2 - Swap in a 4.6L 3v, drive it around for a bit, and decide where you wanna go from there. You should be able to do 450-500 RWHP on the stock block with turbo without much hassle (as long as you don't get an engine with 50 million miles on it). Later on, you can consider building that and bumping up the boost, after doing supporting mods.
Regardless of what you do, just keep in mind, you WILL blow the 4.0L stock 7.5" rear end. Even with a Detroit TrueTrac, I still smoked it the first time I got good traction at the track. I've seen it happen on the street, too. The 7.5" is just hard to make strong. Gone are the days when brand new 8.5" GT takeoffs were common at nearly every reseller. You'd have to get one used now, and that is also a mixed bag (depending on how the car was driven).