If you don't really want to hack your Kindle, just be *able to*, at a later date, you can simply install only the jailbreak. It won't change a thing, except enabling you to install custom updates.
But, to answer your questions about the fonts hack:
* It probably won't brick your Kindle
.
* At the very, very, very worst you might have to do a reset to factory defaults (but even in case of crappy fonts, that shouldn't be needed, a simple file to delete over usb and a restart will disable the hack), so backuping your documents folder is a good idea. (Actually, it's a good idea whether you want to hack your Kindle or not
).
* It doesn't overwrite anything in the system, you'll still be able to access both the files themselves, and possibly the default font itself via the [Aa] menu (depending on your hack setup). Switching fonts *won't* be as easy as using the [Aa] menu, though.
Check the thread in the dev subforum for details.