UA Apollo interface to be primarily used as a mixer would be overkill and a waste of money that you could put towards other purchases.
UA has great conversion but its advantage is obviously the the DSP and quality of plugins.
A unit that people overlook but has amazing conversion and a built in mixer/standalone operation is the Steinberg MR816X or CSX (Effects built in)
I would recommend that :
MR816X (firewire 400 ports) Approximately $500 on ebay
Apollo Twin Duo (can be had for under $800 if you shop around)
great micpres and micpre/compressor emulations. So compression immediately following the micpre, great channel strip.
UAD quality plugins and
ADAT optical in (so any audio interface/or micpre like audient ASP008 for more channels)
High quality headphone amp /guitar input
MR816 /Apollo Twin Total $1200-1300.00 usd
Steinberg UR824 $800
24in 24 out on board DSP reverb for recording with or without effects
2 banks of ADAT$
8 Analog in 8 Analog out
Word Clock
2 Headphone jacks
And the Stienberg units support standalone mode (mixer functionality
UR824/Apollo Twin $1800usd
Apollo Quad $2500 MAP