For your size engine, the M90 may be overdoing it. The M62 is suggested for your displacement. The following two links can answer many of your questions.
1) The newest generations of M90 blowers appear in Pontiac GTP, Bonnevilles, and some Buicks. The 94-95 Supercoupes had S-ported blowers which produced more power than the 89-93s. All of these vehicles are common "over our way".
2) I believe there have been 4 different generations, but do not hold me to that.
3) Answered in 1.
4) You can probably find someone willing to ship you a blower and it's components.
5) Stock boost pressure ranges on cars, from 8-13psi, but some people see much more with OD pulleys.
6) Modify how? There's only so much a person can do to a blower. Others can answer this question much better than I can. Magnum Powers offers ported blowers and other interesting tidbits.
7) Refer to 6 and to anyone else that may reply.
8) Yes, the top is a bit restrictive and by putting a raised top on boost can be increased.
9) If you're buying a used supercharger, rebuild it before you use it.