Howdy,
I am a fan of the buy once cry once theory. There a ton of bags out there. Buy quality! Remember, your bag will hold heavy, sharp things and will spend a lot of its life on rough surfaces. I have an Eagle Industries pro shooters bag. If you can find one, it's the ***** kitty. Another good choice is the one from CED. Here is a link
Shooting Bags - CED Professional Range Bag - Competitive Edge Dynamics USA
For the contents, keep it simple. Heres what lives in mine:
Gun(s)
Mags/speedloaders
Holster/mag pouch/belt
Pro gear (eyes/ears)
Ammo for the session (limited to a box or two. More is carried in ammo can)
Dummy rounds
Sectional cleaning rod
Lube!!!!!
Spare parts with appropriate tools (just enough for a field repair/loctite)
Stapler and staples
Timer
Spare batteries for anything that needs them
Paperwork (CoF/Qual sheets/note book and pencil/sharpies/Emergency contact info)
BooBoo kit (Snivel gear i.e. bandaids, sunscreen etc)
Blow out kit appropriate to your training level (GSW fixing gear)
Those are the things that are standard in the bag. Some things like foul weather gear may be added for different ranges. Depending on the lesson plan for the day, specific work gear like pro masks might be added.
As with all advice, YMMV but thats about all I carry. Weighs in at about 18# without ammo.