In the meantime, I just received an email from a reader asking about using a Cisco 1720 router to learn IOS commands and thought I'd share my response here.
I used to own six 1720s and found them excellent for learning, experimenting, and teaching. Are you familiar with GNS3 (www.gns3.com)? It's a free router emulator platform which is used by millions of people to learn and test Cisco IOS commands. You have to provide the router software, but GNS3 itself is free. It supports the following platforms: 7200, 3600, 3620, 3640, 3660, 3700, 3725, 3745, 2600-series, and 1700-series. An easy way to get the software is to purchase one of the supported routers, making sure that the router you purchase has the most recent version of the software, such as 12.3 or 12.4. It's important to realize that on older platforms such as those supported on GNS3, the most recent software available could be three or four years old. Still, for practicing basic IOS commands, setting up access-control lists, and experimenting with routing protocols, the older versions of the software will be helpful.
Several of the screen captures from The Accidental Administrator Cisco Router Step-by-Step Configuration Guide were taken using emulated routers running in GNS3.
Hope that helps.