I always use steel close nipples for my builds. I recommend 2 ball valves and 1 cap. With the cost of ball valves starting to climb again I can't see the value that a 3rd would add. Measure from the top of the elbow to whatever height is good for you and buy the appropriate black pipe nipple. Don't forget that the ball valve will add about 3-4 inches to your height when added to the pipe. The cost of building the intake system with ball valves, risers,nipples and hardware at my Home Depot here in Northern Ohio is $42.00 give or take a couple bucks. The cost of 2 vent kits from Big Poppa Smokers is $53.90. I don't know if there is anywhere else to buy vents for the UDS that would work as well as the ball valve build or BPS kit and I've looked all over for something more cost effective. You can always use the kitchen magnet type vents, but I can't as I make them to order and they're ordered from pictures. I just received the BPS kit I ordered last week so I'll have a better idea about their vents later.
Hope this helps you and good luck on your build.
The early bird may get the worm, but the second mouse gets the cheese.