Studies suggest that the average number of apps people download to their phones and iPads in January 2011 was 60 (
http://www.macstories.net/news/on-average-each-ios-device-has-downloaded-60-apps/). If the trend shown in the graph continues, then by now it should be around 90. Studies on the app store have found that the average app is around 10mb.
So, if you're an average user, you can expect about 1gb of your 16gb to be filled with apps.
For reasons I can go into later if you're interested (to do with base ten versus base two when calculating storage, and how much space the OS takes), a 16gb iPad actually gives the user 13.3gb to play with. Assume around a gigabyte for apps, and you're left with something like 12gb of free space.
In terms of your own documents, the best way to find out how much space you'll use is to take a USB pen, copy on to it what you think you might store on the iPad and see how much space has been used. For peace of mind, double it. Alternatively, assume sizes of something like 2mb for a long Word document and something like 20mb for a decent-length PowerPoint with pictures, and estimate what you think you'll have.
In regards to books bought from the Apple iBookstore, Apple's FAQs state that books are generally 2mb each, though the store does offer textbooks that can be up to 2gb.
You'll probably find that, once everything has been taken into account, you'll still have 10gb of potential space free.
The reason many people will buy larger capacity models is so that they can store music (an album with high-quality audio will be around 120mb), television shows (about 200-350mb per standard definition episode. HD is closer to 1gb per episode), films (standard definition: 700mb. HD: could be 3-4gb), and lots of pictures (assume 4mb per high-quality image).
It's worth noting that some of my figures will be the upper limits of sizes. At the end of the day, though, unless you plan to go media-crazy on your iPad, the 16gb variant should offer more than enough storage.
EDIT: Pretty much what Metryq said - 32gb for lots of media, 16gb if you're just playing with documents.
EDIT2: Oh, well if the company is buying it for you, try and squeeze every penny out of them! Tell them your presentations may involve high quality video demonstrations and the like, and that the extra space afforded by the 64gb model means you'll be able to do everything on the iPad, rather than having to keep swapping things from the PC.