I'm not sure what you are asking here. Are you a neon or cold cathode sign fabrication shop? While it is possible to make a 'mercury' bent tube lamp without any phosphers, it is not the same as a mercury HID lamp that is commonly used in graphics arts machines.
Be very careful with mercury loaded lamps that do not have any phosphers. The high energy UV light (UV-B and UV-C) that is emitted by an uncoated lamp is very damaging to your skin and eyes, even with reasonably short exposure. Even coated lamps can cause problems if you are not careful. I never work around my exposure units without wearing a pair of UV protection goggles. They are readily available from home centers; check to make sure they have a very high UV absorption rate.
I recommend that if you are thinking of using a bent tube lamp arrangement, you might want to look at the sensitivity curve of the particular process you are interested in, and dope the tubes with appropriate phosphers to meet the peak sensitivity.
If you are not going to be using a light integrator, then you will probably turn on the lamp and leave it on for the entire printing session, so warmup time is somewhat irrelevant. With an integrator timer, the warmup becomes somewhat irrelevant also, so generally, warmup is a non-issue, unless you are planning to be controlling the light without the use if an integrator, which I don't like, because it can cause some inconsistancy problems.