I m willing to pay around $800 shipped to Australia
Ditto! I am currently using a modded c64 breadbin with Mssiah (with only one 8580) - but craving more SID channels.
rosch realitycheck 
all incl shipping and taxes, estimated prices
smashTV 260€
fan display switches standoffs screws 55€
JB Weld 13€
Case 35€
Frontpanel ca 130€ ?
stereo pots knobs phones jack 14€
Waldorf knobs 25€ if a bulk order happens
8x SID 8580 ca 250€ if you buy from a trustworthy source like androsid.
that’s 782€
that’s ca 1127 AUS$, without shipping.
that’s 1187 AUS$ inkl shipping.
without power supply
that’s just hypothetical in case you had someone in europe build it.
of course, 800$ shipped to australia, working and tested, incl psu (maybe incl taxes?)
would be a great deal.
good luck 
but why not diy. it may be more expensive but you’re actually learning something.
and it’s fun to build it.