as said drinks are expensive especially around the champs elysees and the main toursist areas , in places like montparnasse they are reasonable and around the student areas , but not the latin quarter arrondisement 7
veggie foods you may struggle , the french tend not to be veggie and althou they are getting better when i went on a works trip ac olleague who was veggie struggled
as for hotels the cheapest ones are near the Gare du Nord and there are a lot of decent ones near the Opera area and again Montparnasse which i would say is the best place as its more french than tourist
there is a brilliant hotel called hotel trancontinental on rue de maine in montparnasse which is in a busy area but not overly busy and also has lots of bars and eating places around it also the montparnasse metro is a major hub so you can get all over paris from there and teh metro is better and easier to understand than the tube
as for weather i havebeen in most months and last time was september and it was very warm and as its a city it gets very humid in the centre
hope this helps and if you need anymore info just shout