Based on your questions, I would suggest you have a local bike shop build the frame.
Also, you need to ask your builder some questions- mainly what the head tube length is and what style headset needs to be used. They designed the frame and will know.

But to answer your questions...

The headtube will be straight headtube 44mm round 120mm or 135mm in length. What length would be best? There is no better length head tube- your head tube should be as long as is necessary in order for the geometry to fit you. If your bike's size requires a 120mm head tube, then thats the length. If it requires a 135mm head tube, then thats the length. Ask your builder what the head tube length is(it may be something different from those two numbers).

I’m looking at zs44/28.6 zs44/30 headset. Would this be the right headset for the tube and fork? The fork has a straight 1 1/8 steerer and the headset you reference is for 1 1/8 straight in a 44mm head tube. I would guess you will need an external headset, but you dont say one way or the other. If you need an external headset, this wont work as it isnt designed for that. This is a zero-stack headset...is that what you need?

Also any tips on how to installing the fork and headset? Use proper tools. You have done this before so do what youve done already.

Does the upper crown sit on top cup? Again, not sure what your frame's design is. This matters to answer your question.

I’ve been told to add spaces. Where should I add the spaces? Under the upper crown? Add them where you added them on the other headsets you installed.