I have just bought an RO unit. My water supply is from a water tank. Hence I need a pump from the water tank to the RO unit to give it pressure. What type of pump is the best sort to get? Do I need to look for anything specific because it is a RO unit?

How much pressure do you have now ?

Do you have a gravity feed water tank servicing your home? Will this booster pump service the entire house or simply the RO unit? Let me know, tom

The tank is at ground level next to the house, so there is no gravity feed and no pressure at all from the tank into the unit. We will use a separate pump especially for the RO unit, completely separate from the current house pump.

I found a pump that just might work for you. It's a Bristan Shower booster pump.(see image) check it out at: Bristan Pumps B-HYPUMP70 2.0 Bar, Duraspeed Twin Impeller Shower Booster Pump 70. - Bathroom Taps and Showers (http://www.taps4less.com/PP/B-HYPUMP70.html?ad=NexTag)
You don't want a full size pump in the cabinet and this one seems to fill your requiements . Good luck, tom