I would add that unless you can find everything you need in skips or have it donated, the cost of buying the materials new soon comes to more then you can buy a ready built item.
That's true. Mine cost me something like £120 for a 2nd hand hydraulic with removable rear wheel hole/plate thing, from a garage in Central London. They advertised on Gumtree as they were replacing theirs with pneumatics ones - the mechanics were bored of foot-pumping the ramps