1. not sure
2. With a 1GB memory stick you're only gonna get like 1-2 PSP ISO files on there. You can have tons and tons of homebrew, emulators, and roms on there though.
3. Homebrew doesn't take up much space at all.
4. DO NOT BUY FROM EBAY. Sandisk are what most people buy and you can just get them at like best buy, circuit city, or amazon. Or if you want an actual sony one like I do then buy one from sonystyle.com
5. Most of the stuff I use are GBA, SNES, and GBC emulators. I also have an alarm clock on my PSP that I use everyday. Other than that there is Bookr which is a PDF reader and a bunch of PSX/PSP games that I have.
6. We can't tell you where to get any games for any system on these forums. Just use google. For all the emulators just download the pack in my signature below. And for specific applications you should just search for what your after on google and see if anything you like pops up. It surely will.
Anyway, welcome to PSPmod. And in case you're wondering all the stuff I mentioned needs custom firmware to run. If you don't have that just use the guide in my signature to get it. Have fun.