No such thing as a first straight razor. You'll learn to use whatever you buy.
Maybe not square point - often accidental nicks especially as you are learning.
Other than that, go 5/8 as they're cheaper than larger ones. I'd go full hollow. And again for cost I'd go vintage. Find one that looks in good nick from ebay or ask one of the very experienced guys here who can often source one for you - I'm thinking @Fergiebilly or @pugh-the-special-one. The benefit with them is they can make sure it is fully honed and ready to go for you. No worried about it not being sharp etc.
For novice, experienced shavers and collectors. The razor looks great, bright and clean with a blemish or two and in very fine condition, had little or no use. It has taken a very fine sharp even shave ready edge.