iMessage works perfectly fine for me in 10.9.2 on my 1,1... might have to do with installation method, but it can work fine. My install was extremely easy with basically no hacked installs or spare boot drive.... just install onto a hard disk from a supported mac and then replace boot.efi file via Finder.
You can install a natively supported USB 3 card for $25, or $49 for a PCIe SATA SSD controller. You won't get top notch speeds but it'll be miles ahead of the included USB 2 or SATA I.
1. I am not running 10.9 on my mac pro for reasons that I would prefer to not have to add a hack to get it to work. 10.7 does not support iMessage.
2. Yes you can add of those (in fact I had one), but as you stated they won't run at top notch speeds, and I had all sorts of disconnect problems. Might have been a bad card, might have been a user failing. Quite honestly, I didn't care.
About the only thing I use my 1,1 MP for anymore, is when I need a bunch of videos transcoded in a short time I can plug it in, Access the desktop from my MBA, load up a bunch and let it go. It isn't as fast as my Mini, my Hack, or even my wife's 2013 MBP, so it really is only when I need to do a bunch (say someone loaded me several seasons of television shows).