I would add to that initial list of considerations (1) the desired energy source (are you looking to use gas or electric), (2) what kind of batch size you're looking to get and (3) what level of automation (i.e. something fairly automated, or something perhaps more hands on, are you looking to use motors, speed controllers, temp probes and PID's, etc). Also keep in mind cooling of the beans, are you going to look to do back to back to back roasts, can you handle the volume of smoke, etc.
I went through the same process moving from a modified popper to a Behmor, then finding myself needing to do larger volumes. I ended up with a design based on a kitchen stove/oven purchased from Craigslist, and while not without it's flaws, it can turn out very nice roasts...I've pushed it as high as 6 lbs in a batch to end up with 5+ lbs roasted. It struggled a bit but could get through it...and then I quickly realized my cooling setup was not robust enough for that! If you are accustomed to Behmor or GC sized batches, 5lbs of hot roasted beans is a LOT to handle!