Ok, first point...
Was the condensation on the floor in a puddle or more of a "haze?" If there is a puddle, what kind of ductwork is above? Could be any one of a hundred things from leaking sprinkler system to some other HVAC system. If it is a haze, then you did the right thing buying a dehumidifier.
Next, asking the Landlord to install building-wide AC is a reasonable request, but as PEP says, it has to be economical and make sense. If you're the only one asking....chances are he won't consider it, so the best thing to do in this case is get together some other tenants.
As for costs and how to go about getting it installed, if there is no central cooling system, that MAY require a major interior renovation. Depending on building size, it can be anywhere from $10k up to over $100k or more. I've overseen the installation of HVAC systems that cost over $500k and had to be installed with cranes. I've also seen very minor systems that simply hook into the existing infrastructure of the building. So it is hard to give a suggestion on the best course without knowing more about the building infrastructure.
As for the tenant next to you, if you think that his business is interfering with yours; raising your temp, disturbing your business, etc, it is definately a reasonable complaint to the landlord. You have a right to operate without interferance from other tenants.
There are commercially available cooling units that will only do your area. Everyone has seen the window units which would be reasonable...but there are also larger, area cooling systems (I saw LOTS of these in Korea...about the size of a refridgerator, but can cool a HUGE space).
If you actually want to know the sizing and exact pricing of the unit, A/C units are rated in terms of "tons of cooling," which is based on the interior volume of the building (a little more complex, but that's basically it). So you need to know the full area, volume, and insulation characteristics. You also need to know the current infrastructure layout (what ductwork exists, etc).
So, a pretty long winded answer, let me know if you are looking for more or if this misses the mark.