yeah, you need a welder, and you need to practice it a bit. a tube bender, a grinder, a cutoff saw, electric drill and drill bits (1/16" to 1/2"), welding gloves, various grinding and finishing disks, socket and wrench set, hammers and screwdrivers - these are all pretty essential to making and modifying go-karts and buggies in a way that looks professional. an air compressor comes in handy too for filling tires and blowing dirt and crud off stuff, as well as a paint sprayer gun (i still use rattle/spray cans).
if you're ready to dive into this and get your hands dirty and learn a lot (research research!), and you have unlimited access most of the tools mentioned above - you can do some quality work.