I have a pretty high end Hanna 3 in 1, a new probe is nearly $300, so I'm thinking of doing something similar, just getting a cheap one (<$100) for general use. Realistically you aren't going to get much more than a year or two out of a pH meter, no matter how careful you are keeping the probe wet... and I'm getting over the price tag.
Need pH (that's a given) resolution and accuracy to 0.01pH would be ideal
Temperature, 1oC minimum, 0.1 would be better - need temp for the ATC
TDS, Conductivity and salinity are all pretty much the same thing (remember that you need a calibration solution for this as well as for the pH) If you have one you can derive the rest.
Been looking at a couple on eBay and a few other places, will let you know which one I settle on, if that's any help.
One other thing, my local Lab supply has 500mL-1L bottles of all the calibration standards and storage solution (well worth getting) use fresh every time you calibrate, you need less than 50mL so it lasts a long time and you don't risk cross-contamination and calibrating with standards that aren't spot on.