The best meter for measuring both dissolved oxygen and conductivity with automatic salinity adjustment is the Hach HQ40d.
The meter itself is item number 1134153
Hach HQ40d Multi Meter
For the LDO probe:
The cap needs to be replaced each year and the I-button which has the calibration data for the new sensor cap. This is item 1161233.
The Hach LDO probe is virtually maintenance free and a new cap is required once a year to maintain performance.
Other DO meters need a membrane/solution change on a regular basis which can be fiddly to get right, and optimal conditions for maintaining the probe.