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As to the size, I think a 6' rake is a nice size for a small frame tractor. I use a Howse 6' rake with the B2910 at my office and the 30hp tractor didn't even know that there was anything attached to the 3pt. The rakes really don't put a lot of strain on a tractor, go with the largest size you can use on your property. I agree with Henro, the 7' is better suited for the B2910. That said, I have a 6' Midwest Rake at the house, it is a nice size for the TC24, but again, the 6' rake pulls easily behind the 24hp tractor. I think it is well suited to the frame size of the little TC (48" wide at the rear tires).
Regarding your question about checking out other brands:
Last spring the Howse rake was $400. The fit & finish of the paint is typcial Howse (not very good).
I got the Midwest rake earlier this month, I paid $415 for their tilt & angle adjustable LR22-72 rake. I believe that Midwest make rakes for Kioti, I don't know if Kioti offer the "22" series adjustable tilt & angle rakes. The LR20-72 is their angle adjustable rake without the tilt feature. The Midwest powdercoat finish is typically supurb.
I do not have the gauge wheel option.