Hi Lauren
you'll hear a lot of rhetoric about what to buy. If your budget is ample you can get a lot of wonderful stuff.
Your list looks pretty good. However you neglected to include thread for your harness needles.
Leather if you plan to tool will need to be veg tanned. The thicker the leather the easier it is to tool imho.
Gum trag is a slicking agent. But you can do just as well with saddle soap sludge (grate a bit of saddle soap bar into warm water, shake until it's dissolved and looks like snot)
As for finishes I tend to go to Resolene, Bag Kote or even RTC.
You'll need to add to your list.
Hole punches
sand paper in grits 80, 150, 200, 400, etc. use this on your edges after you've trimmed and before you slick with Gum T or Saddle soap
A slicking wheel.
Dye. Start out with Fiebings Pro Oil... My fave color right now is Mahogany.
Daubers finish applicators OR
air brush for the best finish for less effort.
Rivet setter/snap setter
Buffing cloths
Pure neatsfoot oil
A good 16oz or larger mallet or maul (at the very least get the largest yellow head hammer you can find at tandy.)
Have fun shopping. Ask the Tandy people for help... most are good about directing you to the right products. But be cautious of the modern tandy craftools (crap tools) they aren't well made. Look at the pro ones there at Tandy... or buy used Craftool USA or.... get on Bob Beards or Barry King's sites.