I would advise against using PVC pipe as a structure frame, the sad fact is that so many peoples plants have been smashed by lights falling from the PVC piping cause they did not shell out the money for a steel frame, the weight of both carbon filters and lights on the pvc can bend and snap it given enough time.
You also need to be careful if buying a second hand tent online, make sure you get a tried and trusted brand, there are many second hand tents that are no name brands made on the cheap that are not 100% light sealed etc. Which makes you worry about the quality of the fabric and the potential fire hazard risk also. Be sure to get photos sent with a lights on outside picture of the tent showing 100% light seal from front back and sides, if this is the route you want to take.
A very easy and efficient method of DIY is to just get all the pieces cut at your local hardware shop, I am not sure about in the USA but in Australia you can ask them to cut the timber for you at the store, if your in a big city like I am and growing is not exactly legal just go to several different hardware stores for the pieces you need rather than buying at one shop. If you are forced to buy things from the same shop do it over several trips so that it is less noticeable, if you buy a bunch of potting buckets, watering system, pvc piping, and a wood frame that is cut with venting holes its kinda obvious your building an indoor garden of some sort.
Good luck and let us see what you end up with