The Ensoniq Mirage is awesome and worth fixing. Been looking for one
in nice shape myself (it has a niiiice analog filter).
The easiest setup imho would be to just run the analog outputs on the
mixer through a Y-cable and into the audio in on the soundcard. From
there you can do all of your mixing/mastering.
Software-wise, I'm not really sure what to recommend as I'm still
using 10-15 year old software for most things. Cubase SL is $300
which is a bit much (OTOH, SX is ~$700) but it seems pretty nice
(debating getting it myself since I'm due an upgrade). It will also
let you synch up your MIDI gear to any audio tracks you're working
with. IIRC it comes with a decent bundle of studio effects and
mastering tools as well.
Right now my PC side of things is pretty sparse and outdated (aside
from synth editing software). Basically I record into an old copy of
Sound Forge, doing as much EQ'ing/compression/spatialization/etc as I
can in my outboard gear, and then use Sonic Foundry Noise Reduction to
take out any low level noise as cleanly as I can (surprisingly I'm not
getting much line noise). I edit the tracks down to remove the
silence and make sure the track begins/ends at 0db to eliminate
digital popping. From there I dump it into a highly antiquated copy
of Digital Orchesrator Pro, which is fairly low-tech but really easy
to work with. Just file--->import--->wav and then everything is
copy/paste/move to get all the tracks lined up/etc. I mix down from
there into a single wav file, and finish the mastering in Sound Forge,
typically using Steinberg Free Filter to sort out the equalization and
then using the built-in Sound Forge normalization/compression to get
the levels where I want them. Then straight to CD.
Jason