When you get the regulator, get one that is adjustable, and can be adjusted for a slow burn or a high burn. You need it to be hotter for burgers, than you will for BBQing a Boston butt or other slow cooked stuff.
Easy to make the burner element, take a inch and a half pipe and saw slits in it, and make a mount. run the propane supply line to the end of it, but not connected to it, and you will have a nice flame all the way across.take a piece of angle and run it over and above the pipe to shield it from grease droppings. Helps with flare ups.
I like to cut the tank lid using only a quarter of the tank, makes the lid much lighter to open and close. As far as the grill surface, you can use mild steel. Best ones I have seen used stainless. If you used mild steel, you can clean it with oven cleaner, wash it out with a hose, light it to dry it, and then rub it down with cooking oil to prevent it from rusting.
Use high heat black paint to paint the grill. When you make the mounts for the tank, make it to hold two tanks, that way you don't have to worry about running out.
An axle out of the rear of a front wheel drive car works real good, and cheap from the junk yards.
Make sure to make a good size shelf, it will be used.
You can permanently mount a oven probe type thermometer to know how hot your grill is. make two exhaust stacks, higher than your head, so the smoke isn't blowing in your face.
I like the ones that have a dampener on them.
Hinges are easy, just weld them on.
Get some thin angle to line the lid, and the surface where it will close. Some extra trimming to make it a tight fit. Takes a little time, but worth it in the long run.
Make drain holes in the bottom.
All I can think of at the moment
Hope it helps