While I agree with bouldersoundguy in theory, I have a cheap compressor that I actually really like. I have a couple of software compressors that rival or surpass it, but there's something inexplicable that I enjoy about hardware compression. It's a fun toy, whether or not it does a better job than a comparably-priced soft comp. There's nothing wrong with finding something that you like, as long as you don't put yourself in the poorhouse or doghouse in the process. Just do some research and make an informed decision on whether or not it's actually going to help you, don't buy one just because.
On an 18i20, you've got plenty of analog ins and outs to loop in a compressor, or you can put it inline with your input signal. I do both. My outboard comp has 2 channels. One channel I loop into the insert of my preamp, which is handy. The other channel I have looped between an analog input and output of my interface, which is more complicated but way more versatile.
Which DAW are you using? I use Reaper, which comes with a handy (yet kinda buggy) plugin called ReaInsert. Slap it on a track as an insert VST, tell it which hardware output to use as a send, which hardware input to use as a return, adjust the in and out levels, and voila. I think that Cubase has something similar. Actually I'd bet that most major DAWs have something similar.
One drawback to using a software loop for outboard gear is that when you render your project, you have to render realtime (1.0x speed Online render). You've got to let the software play in realtime so the hardware can react to it. No more 20-second renders of a 4-minute song. It takes 4 minutes. Or you have to realtime-render any track that uses your compressor, to make it available for other tracks that you want to send through it. With software compressors you can slap on as many instances of a compressor that your computer can handle.
