Let me clear a few things up...
1. The 4 cylinders in the 99's were 2.5L, which is just a stroked 2.3. The 2.5 would be a really easy swap for a 99, but forged pistons for a 2.5 are harder to find (therefore more $$$$). You could just de-stroke a 2.5 back to 2.3 and use the turbocoupe pistons. Any 96+ plus computer can be tuned to work with any amount of forced induction. Just make sure to use a 4 cylinder computer for a 4 cylinder engine.
www.rangersrevenge.com has a guy that has a turbo 2.5 in his 98 I think. You could look there for more info.
2. My ranger transmission had 250K miles on it when I swapped. 15K of boost later it's still fine. You may not have as good of luck. The 4-banger T5 is an easy swap but it puts the shifter like a foot further back. Another option is the small block tranny adapter plate from
www.stinger-performance.com that will allow you to mount any full size ford 5.0L bolt pattern transmission.
3. Your 7.5 would probably be fine with a turbo engine so long as you don't get too "jerky" with it. Otherwise a 28-spline 8.8 from a 4.0L ranger is a bolt in and the 31-spline 8.8 from an explorer is a near bolt in.
4. The 3.0 and 4.0 put up with modest boost relatively well if you have a good tune, but a 2.3/2.5 will be far cheaper, more reliable, and make a lot more power if you're really going to crank the boost up. They just have so much more support than the V6s. The problem with the 3.0L is that there is virtually nothing in terms of aftermarket heads, intakes, cams, etc for it. The problem with the 4.0 is that it's really hard to get them to breathe half way decent. They can make a lot of torque, but they always seem to come up short for HP given their displacement.