If you're in Ohio, computer gauges only.
In KY you can use either, computer or mechanical.
Personally, for the sake of frustration and ease of use, I'd be buying a computer type gauge if it's used in your state.
Buying used is a huge mistake; don't know if it was dropped, properly calibrated in the required timed intervals and if the setup was exposed to harsh weather conditions.
Bite the bullet and buy new go for the brand names; I get the DW&BP magazine from IAPMO regularly and lots of people want a piece of that industry making gauges.
The computer gauges operates 10 to 1 easier, does a printout and makes for easy work to test.
I personally haven't tested in years but I keep my hours up yearly and went after the license for the knowledge base solely.
Hopefully someday the state of KY will enact a provision that every licensed journeyman in KY must have a backflow certification. Plumbing is not a one directional flow system, like it's initial design is built to be.