I have an Indy, don't know much at all about Octanes. They're not really good for much anymore beyond conversation pieces, but they're "interesting" hardware. Outside of IRIX (which is almost impossible to get legally) a few models will run Linux and NetBSD to varying degrees of success. I've run Linux on the Indy and it works pretty well. Not super-exciting or anything, mind you. (The machines are basically roughly equal to a low-end Pentium)
You can use a multisync VGA monitor on an Indy with a VGA->DB13w3 adapter. (It's the same plug most older Sun workstations use.) The adapters used to be available for a song from industrial computer surplus vendors but they're getting pretty thin on the ground now. The Indy uses a PS/2 Keyboard and mouse as well, but be forewarned that it can be pretty picky about keyboards. (It follows the IBM PS/2 signaling standard but it uses a few commands that aren't usually used by x86 PS/2 machines, meaning many cheap keyboards won't work. A "good" keyboard like an IBM Model M or a Microsoft Natural will be fine.)
You can use a multisync VGA monitor on an Indy with a VGA->DB13w3 adapter. (It's the same plug most older Sun workstations use.) The adapters used to be available for a song from industrial computer surplus vendors but they're getting pretty thin on the ground now. The Indy uses a PS/2 Keyboard and mouse as well, but be forewarned that it can be pretty picky about keyboards. (It follows the IBM PS/2 signaling standard but it uses a few commands that aren't usually used by x86 PS/2 machines, meaning many cheap keyboards won't work. A "good" keyboard like an IBM Model M or a Microsoft Natural will be fine.)