Sorry for the late reply as I was busy.
First of all your Phase Query. Single, two and three phase means that you have one, two and three active (Live) wires carrying 240 volts each of current in your house respectively. Current flows back through the neutral wire. If you have 4 wires in your electric meter (1 live wire and 1 neutral wire coming from Wapda pole to your meter and 1 live wire and 1 neutral wire from your meter into your house distribution panel) it means you have a single phase setup. Similarly a two phase setup will have a total of 6 wires in your meter (2 live + 1 neutral from wapda and 2 live and 1 neutral to your home) and 8 wires if you have a three phase setup (3 live + 1 neutral from wapda and 3 live + 1 neutral to your house). Keeping this in view, you can easily figure out what phase setup you have.
Purpose of phases is to balance and accommodate the load of your premises. In my opinion, it is not advisable to put a load of more than 6000 Watts on a single phase (though it can afford more). Continuous heavy loads on a single phase poses serious hazards such as heating up of main wires, short circuiting, fire and even burnt up Wapda meter. I have a big house and have divided/adjusted my load on the three phases. All lights and light plugs are on 1st phase through the UPS. 1 x fridge, frezzer, water dispenser, washing machine, iron and 3 x AC's are on 2 phase. 1 X fridge, oven, water pump and 3 x AC's are on the 3rd phase. Imagine what could happen if I don't balance the load appropriately???
I don't have an idea about your load and requirements!!! A single phase generator will always be cheaper and it doesn't mean that you cant run other phases on it. A single phase generator can run all the three phases of your house. You just need to tell the electrition/installer to loop the generator phase output with all the phases in your house.
If you think a 5 KVa genset can take up the load of your necessary equipment that go for it (regardless of the phase thing). I would suggest you to only keep the necessary equipment (AC's and freezer) on a diesel generator because in the long run Diesel will cost more than gas. You have to change your preference when you switch from Gas to Diesel for the obvious fuel cost. Think about it!!!
2nd suggestion, if you have more load, you should seriously consider to get a three phase meter installed. It's not safe.
Hope it helped.