IMO, If your just getting started, as in no tools at all (not sure what you have available), then your going to have a high initial cost depending what you want to do, the list can go on and on: welder, welding supplies, grinder/ cutting tools, various clamps/ tools, and materials would be the basic. The cost would probably be close if not more than the cost to buy bumpers and sliders. But the satisfaction of building your own stuff is awesome and you cant really put a price on the knowledge you gain.
Before you make a huge investment I would buy a cheap welder at harbor freight
and just try it out. Practice laying beads in any scrap you have lying around. If you like it and with enough practice you could easily make any armor you want.