IANTD has a course that is called Advanced Recreational Trimix that has a depth limit of 160' - perfect for the type of people you mention.
I do not have a set depth limit for myself on air. It all depends on a lot of variables:
- Why don't I have trimix? (Cost? Availiblity?)
- How warm is the water?
- What is the vis like?
- How hard will I be working?
- Any other inherent dangers about this particular dive?
- etc etc
One thing to keep in mind is that He doesn't only reduce narcosis, it also adds other issues (notice I said "issues", not problems). Your deco schedule will most likely be extended with He. You have to start thinking about ICD, among other things. Not that these aren't manageable, just things to think about.
P.S. I have paid for my CCRs in my savings in He multiple times... but I do a lot of deep trimix diving