I wouldn't worry much about the installation difficulty unless you plan on R&R'ing it every other weekend. The downpipe is the only tricky part - the rest is easy.
The larger you go in size the louder it will be. If you're concerned about noise, put two mufflers in - one where the cat used to be and one in the back where the OEM one was.
2.5" is more than sufficient for most anything in my opinion, 3" will cover any possible thing you could do to your car under 300HP. I would not recommend going 4" like it did (see signature). Not that it won't work, just that it's completely unecessary and a HUGE ammount of work.
400 series is fine for diesel exhaust - yes it turns brown, but does not corrode at any significant rate to be a problem in the next decade or more. If you want it to be shiny and mirror like under your car, then get 300 series.
