There are not a lot of choices
Kohler, Gerber and American Standard make a rear outlet, floor mount with Flushmate pressure assist. Of those, I kind of like the Kohler, but they all work well.
Of the Flushmate tanks, either the 1.6 or the 1.1 gpf work fine.
I do not recommend the dual flush by WDI, EcoFlush. I quit selling that tank.