Suggestion, forget about the upgrades for now. They are not needed for a car that doesn't run. Focus on the basics. Get it running and stopping with what it has and enjoy it for a while. Then move on to upgrades as you see needed. The stock brakes will work fine if properly maintained and adjusted. They won't be as good as disk brakes but they will be far easier to repair than installing disk brakes. Just drive it with a little extra caution and insure the emergency brake works.
I suggest you get familiar with my website,
www.pontiacsafari.com. It has the Shop Manual and Body Manual (Fisher Body Service News) available online. You'll find much more interesting and useful information on the site. Get familiar with the '55 Shop Manual on the "Pontiac Garage" page. Learn how to do maintenance work there. That's where you start, maintenance, and you may have a long road ahead of you on doing just that, especially if you don't have much auto mechanical experience.
Regarding relevant Facebook groups, the best to use is the "1955, 1956 Pontiac" group,
https://www.facebook.com/groups/321200353272194. The group given by fyrffytr1 is okay but largely about pre-55 Pontiacs and a bit too "social" to suit me. There are knowledgeble people there but you need to be careful. Pay attention to good advice given by Goatracer1 above. For good answers on techincal issues, there is no better place than right here in this forum. I follow Facebook groups, but this is where I go for Tech advice.
A couple of quick tips. Lug bolts on left side are left-handed and right side are right-handed. Assuming you have the automatic transmission, to put it in Park, turn engine off and put selector in R (Reverse) position. Check all rubber hoses, if they are old, they will need to be replaced. When you remove the gas tank, check it closely for possible leaks and damage. Some radiator shops will test and work on gas tanks. Check line going from tank to fuel pump. You should also drain the transmission and install new oil.
Any idea how long it has been setting without being run? Do you know if the engine is stuck? Will it turn over using a socket with breaker bar on the crankshaft?
Another place for parts is California Pontiac Restoration (CPR). They have a lot of same parts as Ames. You can get catalogs from both CPR and Ames. Or just get to know them online.
55 Starchief, nice car. I hope you get it back on the road. You will enjoy it once you do.