1. I am writing to enquire about THE Luxury Holiday Apartments that you offer.
4. Could you PLEASE tell me whether your/albo THE apartment (pisz mala litera) COMPLEX has a swimming pool? [please...gramatycznie jest ok, ale wole wstawic po "you"]
5. What is the price per week in August (for renting an apartment)? [nie potr. "full price"]
6. Where are/is the [apartments/apartment complex] located? ARE the apartmentS (pisz mala litera) in the South of France?
Nie rozumiem czemu piszesz takim "uprzejmym" tonem. Are you asking for a favour?
"I would be grateful..."
"Could you please tell me..."
"I would be interested..."
It would be fine to include a "courteous" sentence in your first/last paragraph, but other than that, it is unneccessary. Ask your questions more directly.