I always use mandrel J bends so you get a long piece with each bend U bends are not as useful for all the strange angles you may need to make. Just get regular mild steel. You should also get at least one or two staight pieces. They can be welded together with a cheap wire feed wleder and do one small spot weld skip and inch and do aonther around and arpund the pipe until its all welded up and sealed. Dont try to run a bead around the pipe. This will usally burn or warp the pipes.
Dont forget to use couples on the key sections so you can uninstall and repair th system. Waste gate pipe, down pipe should not be welded together in case you need to replace parts and repair leaks.
You can also use couplers to hold the pipe in place while you get the final shape and remove and weld each coupled section at a time. Hope that makes since.
Also for intake pipe only use aluminum pipe. Same deal as the exhuast with J bends and silicone couplers useing proper clamps. Dont weld the aluinum charge pipes. Its a pain in the *** to have them all welded up. Just use the god couplers and clamps. I usally buy a charge pipe kit from ebay. They are cheap.
Hope that helps.