If you're coming from Windows (or Linux), it will take a few days or a week to get used to just the basics, especially the window buttons being on the opposite side. But once you get used to that you'll start to notice how much easier OS X is to use than other OS's.
As for must have apps. iLife and iWork are number 1 in my book for must have's. You can also look at MacUpdate to find a ton of apps you've never heard of, read some reviews, and download them direct from there. (iLife '09 works on Power PC Macs. It includes iPhoto, GarageBand, iDVD, iMovie, and iWeb - forget iWeb, Apple has dropped support for it. iWork '09 is the last version to work on Power PC Macs and has Pages, Numbers, and Keynote, which are Apple's far superior versions of Word, Excel, and Power Point - you might already have iLife or iWork on your Mac from the previous owner)
Keyboards are a personal thing, I think, as for what people prefer. How they feel, how big/small they are, wireless/wired, feel of keys, sounds, etc. It's hard to recommend something and have someone be 100% happy. If you have a large computer store near you they should have many models on display, you can always feel them before you buy anything.
Apple makes some very nice keyboards these days but they're not inexpensive. The wired/wireless models are in the $50-60 range I believe. I used an aluminum Apple wireless model for a while and then went and bought the wired version with the keypad on it because I need faster access to a keypad for work.
There is a company called Macally that makes some Mac specific accessories, including keyboards. That's the only other brand I know of other than Apple that has keyboards with the OS X shortcut keys, which is very handy, especially for a new Mac user to learn which keys do what.
Also keep an eye on Craigslist, people sell (older) Apple keyboards dirt cheap pretty regularly if you prefer a clean, working Apple keyboard on a budget.
