Since nobody else is chiming in here... Any of the above listed tonnages will work, and there are a lot of variables, like, what kind of patterns you're making, and what size stacks you want to work.
I'm personally in the process of building a new 50 ton press, I've used everything from a 20 ton, to a 130 ton press. I started making damascus in 4-6" tall initial stacks (30+ initial 1/8th layers, as an example), on a 35 ton press and a 88lb self contained hammer. Currently I have two 75lb utility hammers, and no press, but I often use someone else's 25 ton press, which is pretty strained by the larger billets. I'm limited atm to roughly a 4" stack. Bear in mind though, that'll produce billets big enough to make a handful of knives out of the resulting stock, depending on size.
Personally, I wouldn't have anything less than 25 tons, *but* if you're just making damascus for yourself, you may be fine with something smaller. I'd say the vast majority of knifemakers out there have 25 ton presses. AFAIK, Uncle Al's press is the least expensive available in that size. I may be wrong.
Bear in mind however, if you want to do feathers or some other patterns that require tall stacks and tool clearance, most of the pre-built presses don't have remotely enough stroke. At least, not the way I do em.
