IP Base is the IP Base IOS image. This dictates which features you will have available to you (and IPSec isn't in IP Base).
32FL is 32MB of Flash... this is how much space you have to store IOS images. As long as IOS supports the size of CompactFlash, you can use just about any CF card... just format it with IOS.
The Cisco 1841 Datasheet contains most of the other answers you are looking for (the 1841 as two 10/100 ethernet ports).
If you're looking for IPSec, you need a
k9 image (either
Advanced Security or
Advanced IP Services). See the IOS Packaging Bulletin for the big picture of how Cisco organizes features into images.