Hey man, first off a couple of questions; How many inputs do you need? do you need xlr or line inputs or both? USB or Firewire? What DAW are you using?
Something like the MOTU 828 would offer a lot of scope for what you want to do; 8 and although i have no idea of the $ price, i'd imagine that you could find one on the second hand market for around $400 (that's based on UK £ price converted to US $'s).
Using outboard whilst mixing does depend to some extent on what DAW you're using, but in principle the idea is you send the signal you want to process out through the audio interface on a different output from your main out (i.e if you're main outputs 1-2, send the signal out on 3-4), plug that into the outboard, and then plug the output of the outboard into an input on the audio interface, set up and new track in your DAW and record the processed sound back in on that new track. if you're processing mono signals, panning the signal to process in your DAW hard left or hard right will only send the signal to one of the outputs on your AI (usually hard left = odd numbered outputs, hard right = even numbered outputs)
if you're using external preamps then, ideally, you'd want an AI with dedicated line inputs. in the past i've found that the combo XLR/jack inputs that offer mic and line inputs on some AI's still send the line input through the inbuilt preamp. if you're running an external preamp into this, you essentially double amp the signal and can quickly lead to distortion. This also goes for the use of outboard compressors, EQ's etc which kick out a line level output.
A patchbay can save a lot of time/hassle for plugging in outboard gear where you have the line inputs and outputs from the AI on the patchbay as well as the inputs and outputs for the outboard on the patchbay, but bare in mind that the cabling can cost more than you expect to set it up (unless you can make/already have the cables), but IMO the initial cost and setup is well worth it in the long run.
In the world of music there's no right or wrong, however there is "tried and tested" and "experimental"