If you could elaborate more on the type of work you want to do. Your title says finish carpentry, but you talk about working on projects in the shop. Are you doing joinery? trimwork? cabinetry?
I can second fred’s recommendation of a decent combination square. It’s a pretty good all purpose tool that will serve you well in many applications. Although if the price of a really nice combo square has you hesitating, a moderately priced machinists square will serve you well too, plus it’s handy for squaring up miter saws, tables saws, etc.
You mentioned marking…do you mean something like a marking knife?
That’s just an example, you can spend as little or as much as you want on one, or even make your own. A marking knife gives you a much finer line and as a bonus when you are working on joinery a good guide for placing a chisel.
You don’t need a marking knife, a utility knife works in a pinch, but I find that the thin blade can wander on you sometimes.
A decent wheel marking gauge can be handy for repeatable markings and long parallel lines.