There are multiple types of resins used for doming, some of them are Polyurethane and others are an epoxy resin. Epoxy will yellow in the sun in a very short period of time where the Polyurethane resins will work fine outdoors.
Speaking as someone who has owned both I would say the Mimaki is more of an industrial machine. Part of this is the way it is built and the other part is that it does not cut. Roland made some compromises to build a print/cut machine, these work great if you do small print/cut runs, but do not...
The Media will not stick to itself on the reel. Prints generally only stick when they go face to face (ink to ink).
The reel will roll up the vinyl onto a emty core so that it does not touch the ground.
Yes you can get one through any Roland dealer. I believe they are around $1,500.
You might want to check the sponges in your capping stations too. If they swell up and touch the heads it will drain the ink from your carts and fill up your bottle and over flow it. I know Ive had it happen.