For sure go with new. If you are on a budget you can always get a test complete kit case of everything you need and slowly work your way up to digital equipment as you go.
For example you can get the Hagen Complete case of test kits aprox $70 (PJ's Pets) Its got your main PH, Calcium, Nitrate, Nitrite, Ammonia, ect. You can always upgrade to digital PH monitors & other devices.
Salinity- there is cheap Hydrometers $15 But you will find not very accurate & best to get a refractometer $125 (J&L) Which very accurate & best long term.
Temp thermometers- way better then the float or stick to the glass ones. Digitals with the prob that goes in the tank range from $15-$30 depending on the display & features.
Hope that gives more idea's. But I wish there was a device out there that could just do it all.
