Rear discharge Vs Center Discharge with pipe to rear.

11, almost 12 years.
But I see a lot of trailers, especially at refineries, mines etc.
Have only seen central unload ones.

 

As said above, if it passes all inspections, should be fine.

 

Not worth buying a tanker, or renting one... Unless you plan on only hauling one product.
Oftentimes products are incompatible. Even though the tanker gets washed, alot of shippers will not use a tanker that had a particular chemical in it on the previous wash.
Now your stuck with a decision... Do you wait and possibly pass up good freight waiting for something that is compatible, or do you swap the tanker out for a "pool" tanker.
Once you swap out, your investment enters the pool, any you don't know who will be pulling it or where it will end up.
If you are leased to a company... It's your equipment your are leasing on, not you personally.
Can become a major headache.
Of course, depending on what you are doing. ie: getting your own freight... It may not be a problem at all.

 

Most modern rear unloader tankers have a pipe running from the back to the center. There is also a U shaped fitting that easily converts it from rear to a center. After it's washed you stow the U fitting and it becomes a rear unloader again

 

At McKenzie just about all of our true center tanks had a belly line. It worked great and the 16 years I worked there, never had a issue. We also carried six hoses on the rack in case we needed extra.
As far as hauling different products, as long as you get a proper washout and documentation you should be good to go.
PS. We also had product pumps mounted between the landing gear and a PTO shaft coming off the back of the tractor. We could unload anywhere anytime.