Can't answer that - imo the only person that can, is the one who has owned it, sent it in to be calibrated, and can tell you how much it's off each year. Just that it has worked and spat out readings for 3 years trouble free isn't enough - without being calibrated, they could be the wrong readings.
I's steer clear of used meters. You'd really want to calibrate anything used as soon as you get it unless the seller just did it, even more uncommon.
Buy new meters, mid line at minimum. If you want to save money on used gear, go that route with dehus and fans.
This is what I have in my meter cases:
http://www.delmhorst.com/Moisture-Meters/Flooring-Wood-Products/Flooring/HT-3000
http://www.tramexltd.com/Our_Products_Details.aspx?ID=13
http://www.jondon.com/delmhorst-bd-10-moisture-meter.html
http://www.jondon.com/hydrosensor-i-moisture-sensor.html
As far as how to build a stock -
The ratio I keep on hand based on what we have needed historically:
For every 1 xl dehu:
10-12 airmovers
1-2 axials
Past that, 1 air scrubber every 5 dehus and respective number of am's and axials
Buy good meters first, hydro-x second (must have imo), dehu's& ams third, wall and floor systems next, desiccant next, heated drying 2-3 years later if you have spent at least 3 times the cost in rentals in that time period and can do it with cash. (I still don't own one...lol).
If you are serious about diving in - spend the money on good gear, buy what makes sense to own first, rent the rest and buy it as you have EXTRA cash in hand. Don't bet the farm - because you can't make it rain.