Wow, lots of questions. Some more info would help. It sounds like you might be starting up a computor recording setup?
How about one of the 'chanel strips' with a pre, some eq, compressor, and digi out?
-Warmth: By adding some harmonic distortion, and/or, rolling off a little high end, or low end bump, could be done with one of the SS pres with the tube-drive option and/or eq. Best yet is just getting the tone right at the source. It can also be done after it's recorded.
-If the pre/AD also has line out and in, you can send the analog signal out and back in to a separate comp or eq, as if it's side chained.
-If it has intentional colloration, by definition, it's not going to be the "cleanest" signal possible, but it sould just be the tone your going for at the time. (Not an issue.) It's just an option either way.
-Is your sound card 16 bit? You can send it 24 and it will chop off the bottom 8. It can still sound good, with a bit more noise/distortion way down in the noise floor. (-80-90 db range?You may never hear it.) You can do that untill you get to 24 bit, (or look for AD with 16 bit option).then you'll want to dither the mix down to 16.
-Does it have to come all in one half-rack? Is there no spdif (coax) in? Will you want to be able to do a stereo pair in the future? It might help if you break the different functions and issuse down into more easilly understandable pieces.
Hope that helps some.