Hello rs24 and welcome to the Starboard Forums!
What you are proposing can certainly be done.
I'll check on who does top level board repairs in your area.
I do not think that routing out a cavity for a mast box or even simply
drilling out a hole for an 8 mm x 1.0 threaded insert is something you
would want to try unless you have quite a bit of experience with repairing
If you plan to use the SUP as a sailboard, in really light winds, you may want to
add a fin box on the bottom to help you stay upwind.
At your weight I would suggest the RIO M as a good first windsruf board.
The Rio M has a huge range of use (both as a longboard and as a giant short
board with the centerboard up).
As far as teachers in R.I. I would suggest heading over to Charlestown (Ninigret Pond)
and look up Ellen Faller from the Starboard Team.
She is an excellent instructor, and has the Rio M (I think) for you to try as well as some
Starboard SUP's that already have the inserts so you could try sailing one.
If you need to set something up with Ellen and cannot find her number send me an email
(firstname.lastname@example.org) and I will put you in touch with her.
One rig on both boards would work nicely!
Hope this helps,