Unless you have more than 10 units with carpets, then a commercial cleaner and its upkeep would probably not be worth the hassle.
A good entry level carpet cleaning machine (carpet extractor) worth having will cost you around 1500-2000 bucks, used. Then you have to have chemicals which are not cheap. I think that rather than investing in a commercial carpet cleaner (which would be the only way to go in my opinion) it would be better to invest that money in better flooring.
I disagree that you would necessarily NEED a truck mounted rig to clean your carpets. Truck mounted rigs are almost always preferred, but speaking as someone who cleaned carpets with portable commercial units once upon a time. a portable commercial machine can get good results - it just takes longer. Red stains and such require special treatments regardless of truck mount or portable cleaning methods.
For practical purposes, I would hire someone with a truck mounted rig since it is questionalbe whether you could depend on someone with a portable to take the necessary time. Also, portable units can bring great initial results when a lot of soap is used, but the residual soap collects dirt right away and the carpet looks bad again real quick.
I think it is reasonable to estimate carpet life at 3-5 years.I would estimate one capeting per tunover myself, but I would not replace with carpet anyways.