1. Overall there tutorial is a great help for a basic build but there can be a few items which can be a little off, always check with the CRL's (costume reference library) for what is required for higher levels.
2. Ideally the paint should have a gloss finish, even if you can only find matt paints you could add a coat of gloss clear.
3. These are the standard paints which most use, you can use something else as long as it is close to these.
4. You can build with what ever tools you like, many of us find a Dremel can save some time trimming, personally I use 3 Dremels with different tips, I'm lazy and can't be bothered swapping out the tips, using scissors is perfectly fine.
5. I can sit in armor but it isn't pretty getting in and out of that position, I have seen some cut the cod which allows a little more movement, this is done behind the belt so the cut can't be seen. I would advise though to seek higher level approvals before making any modifications, once approved you can do what you like with your armor
This is a great thread full of info if you haven't already found it.
Good luck with the build