TDS an EC are the same. You could get by with out one

The PH meter is what you really need and pick up a tds when you can.

Here is a shop I found the cheappest price for a good little PH meter.

pH METER (Hand Held) Milwaukee pH600

don't forget to get a bottle of calibration solution. You need the 7.00 or 7.01. It is very easy to calibrate and needs to be done regularly.

I get 2 or 3 ph meters at this price just so I am never out.