I think you will find, as others have said, that the heat is really not an issue...in fact, IMO, it blocks some of the heat coming off the road and blasting you in the face while going down the road at speed. If you get hot at a red light, just flip it up for a minute or two.
IMO, there are a few things to look for in a modular helmet:
1.) That it fits the shape of your head...this is true for any helmet, but particularly for a Flip/Full Face.
2.) That it is fairly well ventilated
3.) That it has an internal sun shield that is easy to operate with one hand
4.) It has a removable liner so that you can wash it from time to time (wearing a Buff helps...or a do-rag)
5.) That the outer shield has detents so you can adjust the opening to several different positions
Depending on the shape of your head, if I were buying one today, my choices, in no particular order, would consist of:
Nolan N-104
Shoei Neotec
Schuberth C3
HJC (their top of the line model)
Shark Evoline 3
The Shoei Neotec and Nolan N-104 have generally received the best reviews with regards to ventilation. But that is going to be limited to some extent on a bagger. But you will get some ventilation from the top vents on the helmet. In addition, those get some good reviews for comfort and the sun shield coming down far enough to not be in your line of sight. They are also easy to open/close.
If you want to read a lot of reviews on helmets, go to:
http://www.webbikeworld.com/They review all kinds of gear there, and are generally up to date on the latest and greatest stuff. They also have a good article on head shapes, shell sizes, getting a proper fit, etc.
A nice source, IMO, for helmets on line is Revzilla.com They have good prices, do their own video reviews, and have an exchange policy that is excellent.
As others have mentioned, once you go to a flip/full face, it's hard to go back to anything else for a number of reasons.