Reddit mentions of Blem Cosmetic Second PEC USA 4R 4" double machinist square acc +/- 001" per 6"

I assume you have not used hand tools much.

The first thing I would learn how to do and invest in is how to get your tools sharp, like really sharp. I mean shiny flat back and edge razor sharp. Without really sharp tools - hand joinery is not fun and much harder than it needs to be.

There is the scary sharp method, I am sure u/Peniceiling's suggestion works great, I use Shapton Stones, there are as many ways to sharpen as there are woodworkers.

But you will need to research and invest time in some way of sharpening and get good at it.

A simple Honing Guide can make things easier when you start.

This Small Rip Dozuki B. was recommended by the instructor I had for a hand joinery class last year as the best bang for the buck they knew of - I found it works great.

I'd get a 1/2in Chisel. When you get the chisel, it will not be sharp. You will have to spend time tuning it up.

A Mallet to hit the chisel.

I personally prefer a round marking gauge. This has to be sharpened just like your chisel so it cuts the wood fibers evenly and cleanly.

Small Sliding Bevel gauge to mark your dovetails.

A good quality double square is invaluable. Best $ value I have found for one of these is here.

A 2.0mm lead holder with sharpener is very helpful. Lets you mark into areas a regular pencil will not fit.

That would be everything I can think of to do all half blind and through dovetails, finger-joints etc.

A drill(press) and a couple more chisels sizes will get you mortise and tenons with standard 4/4 6/4 and 8/4 stock.

Good Luck.