I think two tons of pressure would be a bit overkill for letterpress! Pulling an impression doesn’t require all that much pressure. The heavy debossing you see on most letterpress printed objects is actually a fairly modern trend, and most older letterpress printing presses needed to be adjusted in order to achieve this effect! I’m sorry I can’t give you more specific figures. I haven’t done any letterpress in quite some time and I don’t have the brand/model of the presses I did use handy.
As for press criteria:
Price (In a perfect world, this would be a non-issue and thus the least important aspect!)
Ease of Use
Ease of Registration
Size That It Will Press
Pressure (This is a bit tricky because while a nice deep impression is lovely, a high amount of pressure could be disastrous for type, plates, and other printing blocks.)
Another very very important factor would be ease of clean-up and press maintenance. I have wanted to get a table-top press for quite some time, but I have to admit that I am a bit intimidated by the prospect of having to maintain one (especially a used press that is no longer in production!). We had two large letterpress presses in my university’s print studio and whenever they broke down it pretty much crippled all letterpress operations. Since the equipment was so specialized we had to wait for a repairman instead of being able to troubleshoot on our own like we did for the various other printing presses. Being able to repair my own press would give me peace of mind and allow me to get back to making much more quickly (and probably save some money to boot).
For paper registration, I think pressing the paper onto the plate would probably be better. Personally, I would be worried about trying to lock up text/the plate if I were to have to press it onto the paper. However, it might be difficult to get the paper to lie flat for printing and not flap up or shift. With the press I used, the paper clamped onto a drum that was rolled back and forth over the press bed. You could hold down the other end of the paper while printing to ensure that the paper would meet the plate correctly.
I am very much a novice at letterpress printing, but I hope that I was able to help in some way!