In general, most forging presses tend to fall into the 20-30 ton range. When it comes to a forging press, it's a totally different set of requirements versus a typical machine shop type of press...... a forging press cycles through far more repetitions than a typical shop type press ever would.... and then you must also consider the heating and cooling cycles the press must endure. Long story short, a forging press must be super heavy duty and well thought out to last. Personally, I consider anything less than a 6" diameter cylinder too small.....to explain.... I've been in shops with forging presses made from 3-4" cylinders....that were running upwards of 4000psi..... that's nothing more than a time bomb waiting to "go off". At those pressures, a pin hole in a line would cut through flesh like a light saber! I simply refuse to work on forging tools like that.
Rolling mills are nice tools to have, but they are not the "bee knees" that many think they would be..... forge welding with a rolling mill often produces more failure than successes....... and when it comes to drawing out damascus, they will make any pattern into varying degrees of "chevron".....just due to the way they act upon the hot steel. Now, don't get me wrong, it's a nice to have machine, but it's not a "cure all".
Both machines are very nice to have in the shop, but they also come with learning curves that most inexperienced individuals are not aware of. The key to either being viable in a shop is that the machine be well thought out, and well built (to handle the rigors of their intended functions). While I've seen others use log splitters converted for forging, I personally would never go that route..... log splitters are just not built to endure a forging type environment...... and you're taking a huge set of safety risks.
The amount of "oomph" is a matter of individual taste. You can get forge welds set with minimal tonnage on a press...... but then you'll need more to actually draw out or "move" the steel. Personally, my forging press hosts a 8" diameter cylinder, puts out 80 tons of force, with a "structure" that weighs in at approx. 2,800lbs, and is powered by a 4 cyl Kubota diesel power unit...... but I also have had 30+ years of experience with forging presses, and often forge billets larger than others would ever consider.