You can also use a ZIF G4 without issue (once you've set the motherboard jumpers to the correct speed, same as with a G3 upgrade)
Absolutely.
"A slightly less frustating crawl" is probably realistic. No fault to Classilla, which is a nice piece of work, but the internet just is that demanding these days.
In my experience the FW/USB combo cards work without issue, even generic PC-world ones. Stick to OHSI compliant USB chipsets and Oxford FW, and you should be off to the races. I have no experience with the Ethernet/USB/FW combo cards, so I defer to trag's note above.
If you are 100% sticking with OS 9? Probably not. But max it out to 768MB if you plan on using OS X or any kind of unix/linux ever.
I notice you haven't mentioned video upgrade.