I need information about these type of tools.
I believe that most of the better articulated systems have some kind of bearing, but lots of guys have built 'em without.
The Trent Bosch one is pretty heavy
View: https://www.youtube.com/watch?v=RxWJAdWLn_Y
If you have a good drill press you can probably build a perfectly serviceable version of the Monster type. I'd use Stripper bolts ( AKA Shoulder Bolt) to make the hinge points. Stripper bolts are hardened and precision and come in very nicely controlled lengths.
http://machinedesign.com/fasteners/basics-shoulder-screws
You will need some decent drill bits. Check out DrillsandCutters.com.
Start with a 1/4" drill and then drill 1/64" under-size and then drill with the size you want and you'll get better holes.
I'd make the arms out of 4140 steel bar stock. It's harder than cold or hot roll, but if you want to weld it, get A36 steel. 4140 requires pre-heating and ramping down from 600F or the welds all crack. A36 Steel welds like buttah. Free Machining steels tend not to weld well if at all. But they are free machining. And they cost a lot. Use a sulfur oil for cutting any steel. A great cutting oil is sold at any BORG under the Ridged brand name for cutting threads in plumber's pipe threading machines.
Check out metals Depot and Speedy Metals, prices can vary.
I am guessing you haven't seen the Kobra articulated system. It is very seductive.
Having built a slightly heavier version of the Kobra and having used it I can tell you that the Jamison system has it's appeal if for nothing else its direct simplicity. One thing my unit does (because it's big) that I've found useful is that it reaches around the tail stock so I can use it with the tail stock in place on the work. I don't think the Jamison system can do that. At bare minimum; you'd need a good metal lathe a drill press and a welder to build anything like the Kobra.