You can perfectly well use the 2500 series tank for inversion processing, I've done 4x5 that way in mine, and it works well. The downside is that it uses quite a lot of chemistry, they used to print the capacity right on the side for inversion use, but IDK if the tanks are still printed that way.
It holds 1200 ml or so for inversion use.
You might want to reserve the 2521 for 4x5 and get a smaller tank, Jobo, or otherwise, to use for smaller formats.
I understand that you can also use a Beseler or similar motor base, using the tank on its side with rotation it can work with 270 ml, as stated in the description. I happen to have a Jobo processor, so that's generally the way I work with it, but doing so isn't essential in the way that B&H implies. You would want the magnet type though, if inversion is your plan, as that one comes with a lid that has a cap, as illustrated in this link
http://www.bhphotovideo.com/c/product/25558-REG/Jobo_J2521_2_Reel_Tank_with_Magnet.html. The magnet part isn't meaningful for you if you aren't using it in a Jobo processor.
David makes a good point, if you're just starting out, and only processing a few sheets at a time, tray processing works well. It does require that you have a darkroom, or a room that can be made completely dark for the duration of the processing time, however.