Although everyone calls them dremels, Dremel is a brand of what's called a Rotary Tool. I think Dremel was the first to market them, but I dunno. either way, Dremel is the most popular.
I believe that Craftsman and Black&Decker make similar tools.
As for Dremel brand tools, there are a few options. The cheapest are the cordless, which if you plan do do something like a window in a case, then you want to avoid the cordless. I would honestly never buy a cordless tool except for a drill. After the cordless, there are a few options. There are 3 sets for the variable speed model. The basic set for $50, the flexshaft set for $70, and the right angle set for $100. The basic set comes with the rotary tool, a case, and some bits. The flexshaft set comes with the rotary tool, a flex shaft, a few more bits, and a better case. The right angle set comes with the rotary tool, the flex shaft, the right-angle attachment, yet more bits, and an even nicer case. I think they all come with the router attachment.
I just finished my window with a Dremel....my design had a lot of cutting, and it is a steel case, so I used a LOT of cutting discs. I think I used a total of 9, but for the first 4 or so, I didn't know what I was doing, lol. So I probably would have used 7, or even 6. Anyway, count on using a few of them. You want to use the Fiberglass Reinforced if you are inexperienced with rotary tools. The regular cut-off discs have a nasty habit of shattering if you put too much force on them in the wrong direction....happened to me, heh. scary thing.