The 718 is for me, the "Goldilocks" choice - tough and simple enough, with enough on- board features and instrumentation to enhance the Shack's utility. It is a "Knobs with buttons" style, which makes operation fairly simple and straightforward, but with simple menus for less-used functions.
If You are gonna buy used, look for fairly recent model, or one that Owner added the DSP option. New ones are being sold with that option installed - (worth over $250.00). TXCO option (high stability Temperature controlled Crystal) is a plus in an older unit, but is not necessary for OLIVIA Digital mode. If you buy a used unit with Voice Synthesis option, You can sell that and recover some of the purchase price, as they are no longer manufactured, and are cherished by sight impaired Hams.
Brand New, from Amazon, units are going for circa $650.00. Used, have seen them circa $400.00. Might be worth buying New?