Decided on a Roland.. which vinyl?

I suggest you contact someone like MDP Supplies | Sign Vinyl - Application Tape - Sign Making Accessories I dealt with them all the time and if you have any questions or problems just give them a call and they are more than happy to advise and help. I mainly use their hotflex which is cheap by the roll or log as they call it. They do vinyl for all sorts of uses.
Mike
In Morecambe

I have had good luck with sisser. But the only way to find out is to try them all.

I lie siser easyweed.
I would recommend using one source for vinyl where possible to help simplify times/temps as they should be the same for the most part.

I've used siser and stahl, some korean vinyl, and lean towards siser