You want to make sure the Indy's do have a secondary cache since
the entry models shipped without one.  The processor spec should
thus read R4600SC and R5000SC, with `SC' for secondary cache. 
Without it, audio synthesis is about 5 times slower.  Yes, they
have quad audio.  Regarding Unix: you also may want to make sure
you get the full Development option for the C compilers.  ^.2 is
a nice and stable OS.  I actually prefer it to later releases 
since the audio kernel does automatically copy analog output 
through to the digital output, that is you can watch your synthesis
while listening to it.  There is nothing in 6.5 audiowise that
really helps you with csound.  The newer compiler, of course are
much better.

Hope that helps,