I can't see the link because I'm in school. I'd imagine you could find a blade for a dremel similar to the link I posted.
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Link
I use an angle grinder for all of my cuts unless I go to the shop. You do have to cut by eye with an angle grinder. Angle grinders have some weight to them unlike a dremel, so I imagine they would be more stable and get a straighter cut. To slit the pipe you just work from the bottom end up slowly. If you get a thin metal cutting blade, the grinder goes through it like nothing. With the thicker grinding blade that normally comes on them, there is usually an amount of instability and the blade won't cut straight in. The grinding blade will wander on top of the metal since it is not designed to auctually cut.
You could just buy some cutting blades for your dremel. I would imagine cutting higher grade metal with a dremel could be challenging. It could definitely work if you had a fresh cutting blade, but it might be hard to keep the cut straight and even. If I were you I would try this first before buying any other equipment. I have actually bought dremel cutting blades at harbor frieght and northern tool. I've seen them at Lowe's as well
I imagine your link was for a cut-off saw even though I can't see it. Depending on your budget I would purchase a standard 14" cut-off saw. That way you can use it for a large variety of projects. Building jigs for cutting pipe on cut-off saws is rather easy. There are holes for jig mounts on most metal cut-off saws. A lot of people build collector jigs for cutting pipe at specific angles use when building exhaust collectors on cars. Building a sturdy jig for cutting the slist accurately would be rather simple. I'll try to dig up some links later.
Another option would be a die grinder which is the equivilent of a dremel on roids. These can usually be picked up at local pawn shops for next to nothing. However, you do need at least a decent air compressor to run one. Watch out buying used ones as the bearings can be shot. If your dremel works ok, then chances are that this would work a little better. Sorry about the long response, I'm bored in school haha