I was in Bangkok the last 2 weeks of July this year. Lovely! While it is start of rainy season, it only rained a few times during the day. Do expect the odd late afternoon shower or evening shower but they only last 20 minutes or so. Same same for August. I was there first week of September and had alot of rain. Many mega lightning storms that lasted an hour or more 4 days in a row before I left. Thunder and lightning, very very frightening! I think you'll enjoy the low season in July.

Thanks for all the infos and advices 

One good thing about going in the low season is that the bars will be relatively empty, compared to the high season and you can have a larger selection of girls. Though when I went to CIB11, it was always full the last few times I went!! So yes, anytime of the season is great. I myself prefer the low season as relatively less crowds!!  

Any recommendations on where to buy Prep and Doxy PEP in Sukhumvit (lower or upper)? thx

Provide us the details, I need to get my ears lasered lol