Arduino PH Meter for Aquarium - What is the best/cheapest option

Hi

I am wanting to implement an Arduino PH Meter and I know there are a lot of these out there that people have made and some details published online.

Is there a good cheap product(s) that can be recommended?

From what I can see I need an Arduino + a PH Meter board + a Ph sensor normally BNC connector. Is there a specific type/quality I should look for?

Can all ph sensors be permanently placed in an Aquarium? What happens if they are not suppose to be permanently submerged and they are?

I do not have a reef aquarium but a simple Community Tank - about 300 litres.

I have done some searching and there are a number of products that people mention but is there a known good set of products that people can recommend?

Do you need to buy calibration liquid? Or do some support not needing this? or is this bad?

Circuits for Arduino

http://www.ebay.com/itm/pH-Circuit-Sensor-for-Arduino-/200779624796#ht_1673wt_1161 - About $28 + Shipping (stupid shipping price on Ebay)
pH/ORP Adapter - 1130_0 at Phidgets - $30 + Shipping (not yet checked price to NZ)
http://www.robotshop.com/bnc-ph-sensor-shield-arduino-3.html - $38.55 shipping (not yet checked price to NZ)

PH Sensors (BNC)

ASP200-2-1M-BNC pH Lab Electrode - 3550_0 at Phidgets = $25 + shipping (not yet checked price to NZ)
http://www.ebay.com/itm/K1BO-PH-Electrode-Probe-BNC-Connector-for-Aquarium-PH-Controller-Meter-Sensor-/251269399186?pt=LH_DefaultDomain_0&hash=item3a80d2c292 - $9 incl shipping - Quality?

I know there are lots of different products but is there any recommended products from people who have already done this?

Chris

I was also looking at PH meters and came across sparkfun's packages, they may be a bit expensive but you get everything you need. https://www.sparkfun.com/products/10972.