I'm a firm believer of form and function so a cosmetic roll bar was out of the question for me.
Mine is made from 60mm OD aluminium tube. It's 5mm wall thickness so should be plenty strong. I like the look of the slightly thicker roll bar but not too thick.
Definitely look at strengthening the seat belt roll bar mount. I was never very happy with it's design so I added a shear plate. The shear plate is just 3mm steel and doesn't add a lot of weight to the whole thing. It does however make it easier to get it in and out as it's a mountsis a bit spring loaded in standard form. You should also look at welding nuts on the underside of the chassis so you don't need assistance to get the bolts done up.
If you are doing 12" hoops either one behind the driver or a double hoop look at making the 3rd leg structural. I welded in a mount to the chassis that the 3rd leg bolts through the boot floor into.
As for height above the body I think there is a factory height but reckon you are best to sit in the car with a helmet on and make sure the bar extends an inch or so above your head.
Cheers