I have both YSI and Hydrolab (Hach Lange) probes. Both are fine and work pretty well. You just should know that some differences exist between these two suppliers. In my experience, the software available for handling, callibrating and downloading data from the Hach probes is easier and more complete than the software included in the YSI probes. On the other hand, the new YSI probes usually include plug-and-play sensors which are very easy to change in the case of failure or accidental damage (if this happens in your Hach probe, you must send it for technical repair to the supplier which, in my case, may involve around 3-4 weeks without the probe -there is no technical service in Spain and we must send it to Germany-). As regards to the DO sensor, my recommendation is that you ensure to buy an optical -luminiscence- sensor (LDO) which is more precise and easier to callibrate than the former -membrane- sensors.
I have additional probes from Hanna (I don't recommend them, they are not so robust and durable than the former).
i am using WTW Multi-meter model 350i. it is easy to maintain and calibrate, not so expensive.
however, it is very important to identify whether you want to measure salinity of marine or fresh water, since the range of the salinity probe can vary!
The probes from Hach have several default approaches to calculate especific conductance, salinity and TDS from coductivity measurements. They range from salt-water to fresh-water, which makes these probes valid for a wide spectrum of salinities (from natural alpine lakes with very low salinity to very concentrated environments with ionic strength close to saltwater). I am not sure if the other probes do have this option, but it is rather convenient.
You can try the ion-selective electrodes like the ones manufactured by JENWAY. Single and multi-parameter probes are available. They have to be used with a suitable ion-meter. You can visit: www.jenway.com and dealers are available in India.
Recently we buy a multiprobe of HANNA Instruments. Is a very good multiparameter, with sensors for pH, temperature, conductivity, TDS, ORP, dissolved oxygen, turbidity, salinity and atmospheric pressure. They have with connection of 1, 3 or 10 meters. Is easy to use and keep in order. An not very expensive.