I think you will find it difficult to find metric sized tubing over in the States, same goes for imperial size here in the UK, not that I lament that- imperial measurement was out of date 200 years ago, why do you guys hang-on to that antiquated system, when the rest of the world saw the light decades ago?
If you get endcaps from over here, they're likely to be sized for metric tube. Metric sized lexan tubing is available from UK and european retailers, but be prepared for a price shock- it's lot more expensive over here than in the States, that's why the clear modules from the UK tend to be machined from acrylic/pmma, and if you want something you can bounce off concrete, we use PVC pipe, which is usually opaque but dirt cheap.
If you go larger diameter, you will probably have a lot of the module above the surfaced waterline. That will quickly ratchet up your ballast tank displacement, so bear that in mind.
Engel tanks pull strongly up to about one atmosphere (not far off 90 pounds of force on a 70mm diameter piston), they will keep going beyond that, but will labour and pull quite a bit of current, and you need to consider the hull seal integrity. If you can keep within say half an atmosphere, I'd say you're good to go.