Jack Brock
I've been reloading for over 47 years and my comments are below.
Yes get the Hornady Cartridge Case Headspace gauge, this single case gauge will measure all your fired and resized cases , it will be cheaper than buying a case gauge for every caliber you have and be more accurate.
Lock-N-Load® Headspace Kit With Body 1 Each
"NO" do not get this gauge, as long as you have a set of vernier calipers and use a modified case to just hold a bullet you will be able to find the lands and adjust your seating depth. This is the old way and it still works today and you are going to be adjusting the seating depth to find the "sweet spot" anyway.
http://www.hornady.com/store/Lock-N-Load-OAL-Gauge-Straight-1Each/
If your rich and famous and own shares in Canadian tar sand then buy both of them, BUT many many people have problems with this gauge, they pull out their hair and go back to the old way.