I have a p-valve in my suit. I have used diapers in the past, but found that they leak around the legs.
The one time I had to go, and didn't have a p-valve or diaper, was up in Lake Superior and had a good 35 minutes of deco left to complete.:waterwork
I went to diapers after that and switched to a p-valve as soon as they came out. P-valves are one of the greatest inventions next to the drysuit zipper.
A well fitting suit is the most important thing to get after mask, snorkel and fins.
It is the one thing that needs to fit your body exactly to work correctly. There is a saying in the dive business: a rental suits fit everyone, so it actually fits no one.
You can buy a very nice wet suit for much less than the price of a reg. Regs work on any body size. Most rental regs work just fine for recreational dives. If your diving is so involved that you need that high performance a reg, then you need to buy all of your own stuff and skip renting, or slow down a bit and gain some experience slowly.
After the wetsuit, or drysuit
, I tell my students to look at a good BC, I actually prefer a wing, because they are the second hardest thing to fit to your body. The last things to buy are tanks. Any rental tank will work with almost any type of gear set-up as long as you don't have differences with an A-clamp vs. DIN valve.
Start with gear that touches your body the closest and move out from there.
I know where your coming from about buying gear one piece at a time. I started diving when I was 14 and had to cut a lot grass and baby sit a lot of little brats, as in the kids that no one else in my neighborhood would sit for, before I could afford my first wetsuit, BC, tank, ect. I bought things in stages and looked through the want ads for as much used stuff as I could find. Now that we have Ebay you should have a much easier time of it.
Jon