I’ve only been riding in whistler in September. On both occasions the weather was lovely.
There are a lot of hotels there and they are relatively inexpensive in the summer. Many of the rooms have a kitchen in them, so you won’t have to dine out for 3 meals a day. Washing machines and dryers are also common in the rooms, so you may not have to take tones of kit and clothes. I use trivago for hotels.
Get a box from a bike shop to pack your bike. I always take my lid and camel bak as hand luggage when flying with a bike. A DH bike is best for the park. Rent one if you don’t have one. If you want a specific or fancy model, book in advance. Even if I wasn’t renting a DH bike, I’d rent a full face helmet and some armour. At least for my first day.
Read up on technique for jumping and berms and drops. With a bit of theory you will be more confident. There are trails from very easy to (in my opinion) very scary. I’m no free rider, but I love whistler.
Don’t forget to book a transfer from Vancouver. I think it is Pacific coaches that operate a shuttle. Check their timetable, it may be worth considering it when booking flights. The checkin for the shuttle is just by the arrivals.
Worca publish a map of the trails outside of the park. It is available in bike shops.
Have fun.