Hi Michelle,
As far as I remember this holiday is half board with some lunches included as well, so you don't need much spending money - I always pre-book all the excursions I can, to cut down the need to bring extra money with me. I would say around £500 would be plenty. Be sure to let your credit card company and bank know you are travelling there, so you can use your cards while you are away.
I had an upset stomach for a day, but wasn't too bad. I've learned the reason for this is not badly prepared food or poor hygiene but just that the food abroad contains different bacteria to the food at home and depending on your own system you may or may not react to this. I was fine on the Chile, Argentina and Brazil tour but had a mild stomach upset for much of the Costa Rica one! On our India tour, the one vegetarian in the group was the first one to feel unwell! I suggest you take a pro-biotic such as Boots Digestion Support Travel pills - I find them helpful.
Don't worry about covering up to avoid mossies, they can bite through clothes anyhow! Just make sure you cover yourself liberally in DEET (50%) and you should be ok. Having said that, I don't think I saw a single mossie when I was in India.
Have a brilliant holiday!
Cheers,
Bob