The weather will be very warm but not too hot. You'll need to take a light jumper for the evenings because it can get a bit chilly, especially if you've been in the sun all day.
Mosquitoes - use a repellant eg Autan and use plug in repellants in your bedroom. However you shouldn't have too much problem with them at the end of September.
Which resort are you staying in? I know of more than one Maloua apartments.
Products you buy at home will most probably be more expensive over there so take any brand name shampoos or shower gels you may want. However if you're not fussed about what you use then it can be cheaper.
Have a wonderful Holiday !!! xxx