1) Yes, you can transfer over everything. iCloud might not be an option since you are running an older OS. In that case, I would get everything into Dropbox and simply install the Dropbox app on your iPad to sync everything.
2) I am 99% certain that iOS does not use text reflow. Check the Apple site's discussion forum for the skinny on that topic. That said though, I have had absolutely zero issues with reading a whole boatload of different document types on my wife's new iPad. It's a better experience than I expected, actually. She's actually about to get rid of her Nook because epubs work so well on that device ;).
3) wifi refers to a type of wireless internet. There's no extra charge for utilizing wifi on your iPad unless you happen to be attempting to connect to a paid wifi network at some coffee shops or airports. Most folks are using free wifi access points though, such as their home broadband network (Cable or DSL internet provider, for example).
I'm considering buying and IPad (or Mini) but insecure about a few areas: 1) will I be able to "transfer" pdfs, .docs, and other text files from my MacBook (under 10.4.11) to the new device and if so, how? 2) comfortable reading of pdfs is a big need for me - does the iPad use reflow? and 3) I know zero about wifi - what's the cost of an IPad or Mini beyond the the device itself?