A good height i suppose a little bit on how tall you are and whats a comfortable position for lifting heavy objects onto.
I did a bit of research around what my desks around the home are, what height other plasma / fab tables are etc. to drive my decision.
I would think the size of your rails should be ok, but as for the rusty bit, I would assume the Y rails are your source of truth and should be as smooth and flat as possible, at least on the upper side where the wheels sit.
As for the slat spacing, that all comes down to the piece sizes you want to put on your table, if you want to work with smaller stuff, you would need smaller gaps so it doesn’t fall through.
However the smaller gap you go, the more the slats will get burnt by the plasma and the more $ it will cost to build and replace, so it’s a balance. Most of the bigger plasma tables look like they have somewhere between a 100-125mm gap between slats.
I would not go any smaller than 50mm, which is what I’m planning to go to allow working with smaller pieces.
Also making the distance between slats an easy division of what your table length will be so you don’t end up getting a headache with calculating distances, especially if you end up modifying the design. Ie. if you are working with 1200mm, make it 50 (24 slats), 60 (20 slats) or 100mm (12 slats).
This may be trivial with a fairly simple design like yours but i ran into problems with my design working out extension lengths (just cause i like to make things complicated for myself).
Your design seems a little similar to the crossfire, have a look for some inspiration if needed: https://www.langmuirsystems.com/crossfire
The spacing of the slats on the crossfire is about 3".