We have been installing plasma screens into locations for the last 6 years!
Plasma screens tend to have a life of 3 to 4 years, but this is based on actual hours of use. usually about 30,000 hrs.
So, for my plasma at home, it is usually on from 6pm - 12 midnight - the expected lifespan is therefore about 12 years!! And I will be replacing it well withing 12 yrs!! And in 12 yrs, plasma will be as cheap as chips or replaced by another technology, so dont worry.
The screens we have installed into say airports running 24/7 all lasted 3-4 yrs before leakage occured (bottom corners started to leak a yellow colour) - We binned them and bought new - even in 3 yrs the plasma we had we old technology!! ie. £8000 new, 6 inch thich and a 400:1 contrast, the replacements were £3000, 3 inch thick, 2000:1 contrast!
So, my recommendations:
If its just a TV used on an evening, and you are happy to replace in say 10yrs - dont worry about re-gassing.
Buying new is better than second hand unless it has done little hrs. If you must get second hand find out how many hrs it has done!!
Hope this helps,
Tim