For starters let me say this, it is not necessary to go out and start buying up all kinds of expensive gear.  To start scouts should be borrowing or renting the equipment they don't have.  The one item that should be invested in now is a good pair of hiking boots.  They don't need to be super expensive, but should fit well, provide plenty of support, and have sturdy soles.

That being said, there will be equipment that the scouts will need for each backpacking trip the troop goes on.  A good backpack with a frame and for the summer trip a large enough capacity for over a weeks travel.  I tend to prefer internal frame packs, but external frame packs work just fine.  A light yet warm sleeping bag.  Down is very light and warm, but loses both when wet.  Man made fillers are warm as well, stay warm when wet, but weigh a little more.  A sleeping pad, solid foam or self inflating.  Either is good.  A good piece or set of rain gear.  A poncho is okay, but a rain suit is a little better.  Each scout will also need to start putting some basics together.  They will all need a whistle for emergencies.  It doesn't have to be some expensive rescue whistle, gym whistles work just fine.  A first aid kit, which they will be working on for First Aid Meritbadge.  A store bought one isn't necessary.  A water bottle.  Empty gatorade bottles and the like will work, but are more likely to break open.  Again high expense isn't necessary, but spending a little on a sturdy bottle or hydration bladder isn't a waste either.  As we get closer to doing our backpacking trips, this list will grow, so please check back later for more information.