If you're going to go with the AD-20, just be sure you don't run a preamp in front of it. Your levels will run way too high.
Also, I strongly recommend you modify the knobs. Setting the levels with an un-modded AD-20 is tough in a concert setting because the knobs are so damn tiny. If you have itty bitty dainty fingers, no problem. Otherwise, superglue some washers or Legos or ANYTHING to the fronts of the knobs so you have something to grip while you set the levels.
I recently purchased a used Apogee ad500e for $200 + tax/shipping. Then I bought a 12v battery & 15-pin cable from Pro Digital to power it for about $145, but you can find them cheaper used. You might be able to hunt around for a similar bargain for a great piece of equipment like that. However, unlike the AD-20, your levels will be too LOW if you don't run a preamp in front of the ad500e (could be an added expense for you). Just some thoughts for you.