My only suggestion is on handhelds. Equipment like a commercial trimmer and blower are pretty cheap to buy new. The price difference from used is minimal, and they should be a lot more reliable. Most guys seem to like Stihl, Redmax, or Echo around here. So for the small increase in cost, I suggest buying new.
Buying a used truck, trailer and mower is a good way to get highly productive commercial equipment for a much lower price, without sacrificing much reliability. Buying a used mower from a dealer is a pretty safe bet, assuming the dealer has a good reputation. Another example...I recently had my oldest trailer rebuilt by a mechanic. New bearings, breaks, tires, and then I painted it. I think it is good for another 10 years.
In my opinion, downtime is the biggest potential cost. If a machine breaks, and you can't work, the money simply stops. Having a plan for rapid repair is essential. Some dealers will promise to repair commercial equipment in 24 hrs, which makes a used mower from a dealer even more attractive. Be careful, some dealers just don't care about commercial customers any more than homeowners.