1 not really, you dont need to rev a turbo lump like an NA one so as long as your not trying to make massive hp its not an issue
2 yes, again as long as your not going mad its very much the same tech, your just not spinning it as hard, just keep an eye on temp and fuel mix with an AFR gauge.
3 how longs a bit of string, realistically 100hp is penty, but i know lots of people that end up with 150hp cars they frive most days
4 depends, but probably not, realistically a reliable daily NA car will be limited to about 80hp otherwise you end up with a mad cam, turbo wise you only ever use mildish cams tso you end up with somthing around 100-120hp that you can possibly wind the boost up on to 150hp maybe more. some run upto and over 200hp
5 probably a good idea, or a 998 turbo conversion is realistic with 100 hp