In our apartment we have a pool of size 35x40 feet with 4 to 6 feet deep. I am in charge of it to oversee the agency maintaining it, for last few years so I think I can give you some inputs.
* Usage will be only on weekends.
Even if it is for weekend use, you will need to clean it on a daily basis.
* How often will the water be have to be changed?
We do it once a year. Even then it is not necessary. As we close down pool maintenance during winter so we drain out the water.
* What is the process and equipment for filtration
Filtration equipment consists of
http://home.howstuffworks.com/swimming-pool2.htm
Skimmer: These are suction points at the water surface level. The filter pump sucks in water through these skimmers. These has metal netting filters to prevent any unwanted objects like stones, leaves etc to reach the filter pump.
Main drain: This is again a suction point at the bottom of the deeper end of the pool. It too has metal netting.
Vacuum suction point: This is again a suction point usually on the side wall. Used for vacuuming the pool.
Valves: Skimmers, main drain and Vacuum points each have its own plumbing line till the pump. These will need to have their own valves so that any of these lines can be turned On/Off as needed.
Pump and Motor: Is readily available, we have Kirloskar make of 5HP rating.
Sand filter: This is no ordinary filter. It has a pressure gauge and valves for filter, backwash, rinse etc. The sand in it can be cleaned using the backwash, rinse valves. Pressure gauge indicates the dirtiness of the filter. If dirty you need to clean the sand. After some time (years/months) even after rinsing the sand the pressure gauge will show its dirty. Its then time to replace the sand.
* What will be ball park figure to get the whole thing built with filtration etc.
I dont have idea of construction costs as it was done by the builder.
* What will be the long term maintenance cost? (Any recurring cost)?
You need to get brush, pole, hose and vacuum cleaning set. These can costs around 10k but can lasts many years.
Chemicals like chlorine granules (Rs 200/kg), Chlorine gel (Rs 200/lit), Copper sulphate crystals (Rs 200/kg). Granules need to be added almost daily in atropical climate like ours. Gel and CSulphate as and when required.
Most important thing is cleaning the pool with the brush and Vaccumming is a back breaking job. Unless you are very healthy and with time in your hands, it can quickly become once a week job from daily one and detiorate the pool water quality.
All prices are indicative, from what I remember. Please correct if these are wrong.