Hi tonero,
To run a vinyl cutter, it needs vector information. It can't use raster information. (mouse over or click on those two highlighted words), so you won't be abl eto cut anything that you design in Photoshop. For vinyl, you will either need to have Illustrator or CorelDRAW (or another vector program), or design in your plotter's software, but usually the software that comes with vinyl cutters is not all that great for designing.
What plotter are you thinking of? If you're ONLY planning on cutting vinyl, and NOT inkjet transfers, you would be fine with a cutter known as the Copam 2500 (I think from USCutter).
If you think you may eventually cut transfers, you need a slightly more sophisticated plotter. If you're set on a 24" plotter because you think you might like to cut vinyl for signs sometime in the future, I can't recommend the Graphtec CE5000-60 24" cutter enough! We have one, and it will do anything that the more expensive Roland GX-24 plotter will, and when you count the cost of a stand (which the Graphtec CE5000-60 comes with), the Graphtec is $500 less expensive.
If you're not planning on cutting vinyl for signs, you would be fine with a slightly smaller plotter like the Graphtec CE5000-40 CraftROBO Pro. it's a 15" plotter that accepts media up to 19" wide, which is fine for most t-shirt vinyls. It also has the registration mark sensor, and is exactly the same machine as the larger CE5000-60, just a little smaller.
And I suggest that you buy either of these two plotters from
Specialty Graphics Supply
With them you get a 5% T-Shirt Forums discount when you enter the discount code (found under "View Offers" to the left here), free shipping, and 10% off of all supplies for 30 days after you purchase your plotter.
Okay, that's the vinyl cutter.
For a heat press, most people will suggest you get a 16x20 like you said. That's fine. We have a 15x15 and haven't had any problems, but if you just want to be safe, the larger press will prolly be your best bet.
One thing...there's a discussion going on in the Heat Transfers forum (actually two or three now) about "Cheap" heat presses. I suggest you read those threads and form your own opionion on what press to buy. I personally suggest a name-brand press with a lifetime warranty on its heating element, but you need to decide what you're willing to do.
I can't answer your question about the two types of vinyl. I personally don't use that brand of vinyl (but I'd like to get some and try it out). We use ThermoFlex Plus, but even that vinyl isn't without its quirks.
To figure out how much vinyl you need, figure out how many designs you can fit width-wise on your vinyl roll, and how long you will need length-wise, taking your width number in mind. Leave extra, because with most designs you will need to leave a little extra space between your designs for weeding and seperating purposes. Also, give yourself a little extra in case of mistakes.
Other than that your list looks pretty good. I guess the one thing that I would add to that, unless you have your own placement method, is to get a TeeSquareIt (
Tee Square It). That will help you to find your centers and to help make sure your designs are straight on your shirt.
Good luck! If you have any questions, please feel free to ask!
Setting up a business--fresh to this 
