I do a reasonable amount of altitude flying here in Scotland.
I live at sea level, smoke, drink profusely (has health benefits . . some)
Get any MH EDS system, you really don't have to go as big on a bottle as
you'd think. If you have easy access to O2, go "medium" on bottle
selection, and steel for the bottle if weight is not an issue.

I routinely get 30+ hours on a very small bottle and that is based on
wearing the system at the launch point (at sea level) to counter hangovers,

then keeping it on all flight (as it is zero hassle and I'm a lazy
b'stard)
The cannula system works very well up to fL200 and allows smoking whilst
flying . . (I'm not at all sure it's rated for it, but it does work)