There are all kinds of tricks that help with transporting dogs in the heat. I have a lot of experience with this as I have transported dogs from Socal to east Texas 20+ times. Long road trips like that are a different beast though. But if your trips are 2 hours or under a good crate with a solid top to keep the sun from shining directly on the dog with as much side ventilation as possible and a portable water dish fixed in it should be more than fine. And the first few times you do this check on the dog regularly to see how they are handling it. Depending on the dog it might not be a problem at all or they could be struggling, you need to watch them close at first to learn your dogs. Also you can lay a towel in there for the dogs comfort and if your dog doesn't mind it you can wet the towel though some dogs do not like to lay on wet towels.
As far as what temps you can work dogs in, you will get a lot of opinions on this so I will give you mine that I have formed through hunting hounds and curs for many years now on bears, bobs, lions, and hogs. It depends on your dog/s and what they are acclimated to. That is the key, you cannot take a dog used to moderate or cool weather and then hunt him/her hard in the heat, you will kill them. Heat will kill a dog faster than most things but if you have a dog from a line of dogs that has been bred to hunt in that area and is fully acclimated to the weather you would be surprised what they can do. I would say this if you love your dog error on the side of caution and don't push them too hard or take unecessary risks. Also if you know your dogs and what you are looking at you should always be watching them and can spot when they are starting to get hot. Happy hunting!