Hi RickOoi,
The way I do it is to calculate the square footage of my walls.  60-70 square foot would need around 1 litre of paint for a single layer.  So you could get an approximate amount of paint needed from there.  It is hard to tell how much paint is needed for 18x65 size house as the number of walls may have different number. 

Nippon, ICI and Jotun are good and popular brands and each brand have cheap, medium to high cost paints depending on features. 

Features:
-washable
-covers hairline cracks
-low VOC
-fungus resistant
-UV protection (for exterior)
-low sheen
-high sheen
-matt finish
-just plain water based paint

Things to consider:
-Sealant needed?  May have to check the condition of existing walls.
-Features required?
-Budget?
-Colors?

If you are getting a contractor to do it, they may recommend the paint to you, so it would be much easier if you know what features you want.  Otherwise they might ask you to buy the paint and they just paint it, in that case you have to know what features you want and also the brand.  Do some shopping around to find out what features is available and how much those brands are charging for those features. 

If you are going to be repainting every 3-5 years, then getting the economical to medium cost paint would be more worth it.  Such as Nippon Matex/Matex Gold. 

Hope this info helps. 

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Added on October 18, 2012, 11:39 am

Hi RickOoi,
The way I do it is to calculate the square footage of my walls.  60-70 square foot would need around 1 litre of paint for a single layer.  So you could get an approximate amount of paint needed from there.  It is hard to tell how much paint is needed for 18x65 size house as the number of walls may have different number. 

Nippon, ICI and Jotun are good and popular brands and each brand have cheap, medium to high cost paints depending on features. 

Features:
-washable
-covers hairline cracks
-low VOC
-fungus resistant
-UV protection (for exterior)
-low sheen
-high sheen
-matt finish
-just plain water based paint

Things to consider:
-Sealant needed?  May have to check the condition of existing walls.
-Features required?
-Budget?
-Colors?

If you are getting a contractor to do it, they may recommend the paint to you, so it would be much easier if you know what features you want.  Otherwise they might ask you to buy the paint and they just paint it, in that case you have to know what features you want and also the brand.  Do some shopping around to find out what features is available and how much those brands are charging for those features. 

If you are going to be repainting every 3-5 years, then getting the economical to medium cost paint would be more worth it.  Such as Nippon Matex/Matex Gold. 

Hope this info helps. 

Sealant have to be applied before putting on Nippon Weatherbond.  The thing about Weatherbond or Weathershield is the surface needs to be cleaned and treated properly, apply sealant and then apply paint then there is a possibility that it will last 5 years  .  If you want a confirmation that it will last 5years or more, then I believe Nippon and ICI have a crew of professional painters where they can confirm the paint will last > 5years. 

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Added on October 19, 2012, 7:22 pm
I got this OFF Nippon website 

Surface Preparation:
Remove all loose, defective paint or powdery residues by wire brushing, scraping or high pressure
water jet.  Treat any areas affected by fungus growth with Nippon Fungicidal Wash Solution.  Repair
cracks, uneven surfaces with Nippon ACS Putty or suitable fillers.  Smoothen the putty / filler areas
with sandpaper.  Allow all surfaces to dry completely prior to painting.  Avoid painting when the
moisture content and alkalinity of the walls are still high.  (Recommended painting specification
requires the moisture content of the walls to be below 16% measured by protimeter and alkalinity of
the walls to be below pH9.)

Previously Painted Surfaces:
Remove all unstable paint film, loose chalk, dust and foreign matter.  Make good any surface defects,
clean off and dry.  Spot prime with NIPPON VINILEX 5100 WALL SEALER.