I am about 90% done making my first homemade tent. It has not been field tested, and I am not yet qualified to enter into the canvas/nylon debate, but I will offer up a few opinions on other matters.
1. I found that sewing the tent was much easier than other smaller sewing projects (bike panniers, tool bags, etc) I have tackled. Having adequate space to layout the panels of the tent is important.
2. The most tedious part of the sewing is the second row of stiches on the a lap felled seams.
3. Think ahead when sewing up your tent, try to do as much as you can before joining large panels, it gets more awkward to sew as the panels are joined together.
4. I'm making a hot tent just see if these things are really as comfortable as the conventional wisdom claims.
I've done it on a budget as a 'proof of concept' project. The materials I'm using may not be the best, I only put US$60 into the canvas and nylon material. In my case, the nylon is not breathable, I intend to use a poly tarp for a fly. I'm curious to compare the performance of the canvas portion of the tent to the nylon.
I've cold tented a bit, and know all about frost build up on 'breathable nylon'.
5. Along the lines of low cost, and probably car camping, I'm using electrical conduit for the poles. (3/4" Conduit with a 8' long ridge)
6. PVC: It might be OK for short pickets (<24") sewn into the corners of a pyramid tent, but I don't think it would be adequate for a long ridge pole, unless you went with Schedule 80 in fairly large diameter. To get a feel for the relative stiffness of the different materials, go to your home improvement store and pick up 10' lengths of pipe, put one end on the floor and try flexing them.
Below is what it looks like so far, I probably have about 20 hours in the fabric layout, cutting and sewing.