I built one, and love it. http://www.bbq-brethren.com/forum/sh...d.php?t=133467
It was hard to find a drum of the right size in my area, so if you have the drum already you're ahead of the curve. You need the grates and lid from an 18.5" Weber kettle. Don't put all the intakes on risers like I did - it restricts the airflow. Just put one on a riser if you want but leave all the others close to the bottom. I would use 3/4" - 1" intakes like folks do for the 55gal UDS..
If you don't want to build it yourself or can't find a drum readily, PBC seems to be the same concept already built and on the market.
You can cook a lot of food on a midi UDS. I have 2 cooking levels and one grate for a diffuser in mine. Can easily do 20 lbs (2x 10lbs each)of pork butt, 12-14 lbs turkey, and 2 untrimmed racks of spares (not all at the same time of course) which is plenty for the 2 of us.