Thanks in advance for any input

Couple of decisions I need some help with

My plan is to record a small acoustic ensemble (2 voices, a guitar and 1 other stringed instrument). Right now I have 2 digital inputs to my Daw running Sonar 3x Platinum. I would like to get up to 4 channels of digital.

I would also like to add a small digital mixer for some monitoring, automation and live mixes 

I already have:
several good mics 
2 channel mic pre (Great River) 
2 channels of A/D – D/A conversation - Fire wire (Rosetta 200) 
12 channel mixer for monitoring (Old Mackie – Analog) 
Good Head phones and Monitors. 
Daw and Soft ware
And a good sounding room with treatment

Hears the rub
As much as a would to match the quality of what i already have, I am on a budget.  I have around $1000 to spend - used is fine
I can pick up a used Rosetta 800 for around $700. This does not leave much for a good 2 channel pre. If I buy and good used pre then not much is left for a converter.
Is there a small mixer that I can use for digital mixing, has good mic pres, and has a SPIF out so I can run 2 more channels to the Rosetta, SPIF into my daw

I realize I will take a loss of quality on the mic pres (compared to the 2 channels from the Great River)
but I could clock the 2 extra digital channels from the mixer from/through the Rosetta 

So, What say you. I am open to all possibilities

The Rosetta 800 doesn't appear to have pres.

If you follow the you tube links below, I compare an Audient eight channel pre and converter with an RME.

Your budget is pretty healthy. You should not settle for something you don't love at that price.

I like the guys at sound pure for some interesting perspectives on pro gear.

An RME UCX has built in pres and has SPIDF.