Big giant coil thingie...Are you talking about a packaged Rooftop Unit? That is a single machine that has the AC and Heating all in it, and sits outside. Either on the roof, or on the ground next to the building?

If so then you will need 2 new units, and a curb adapter to make the new unit fit the old roof openings. That is the outside part of the equipment.

Various items also may be required, based on local codes and how strongly they are enforced. Things like a manual fresh air damper, or an economizer to bring in fresh air, smoke detectors in the duct to shut off the units if there is a fire, and some codes even require C02 detectors to bring in fresh air based on occupied conditions.

If you pay the utility bills, then ask for an economizer, even if you pay extra for it. An economizer is a set of moveable louvers that sit on an opening on the unit.
These louvers can be totally closed when your building is not occupied, or if the AC or heat is not running. They can be opened up to bring in "free cooling" (outside air blown in by the unit's fan) when the outdoor temp is 55-70 and your inside space calls for cooling). Free cooling sounds good. But that's not even the most important part. The economizer is linked to your programmable thermostat, and will be totally shut when your space is not occupied, which is more than half the hours of a typical week. By closing this, you don't have outside air getting in your space.

If you don't have an economizer, your code will likely make you have a fixed damper for fresh air. This is very inexpensive to install, but basically is a vent that is always open about a third. Always. Even when its a night or weekend. Leaking in outside are all of the time. Remember how your dad would holler at you not to leave the front door open because he didn't want to pay to heat the entire outside? If you only have a manual damper, then this is open all the time.

The good news is that 7.5 tons is a very very common size. Your mechanical contractor should be able to find units in stock at the distributor if you are in a large metro market. Or they will be on a quick ship factory option at the factory if you are in a smaller area. But you should not have a 4 week lead time or anything like that for getting the equipment. Curb adapters can be fabricated and ordered in in about a week and a half at standard pricing.

If the units are on the roof, then I would expect the whole thing to take a couple of days to be installed. If there are any problems, this might take longer. If you area requires inspections that must be passed before occupancy, this can also add to the time.

They will have a truck with a crane come to deliver the curb adapters and new units, and the old units and will have to be craned off the roof.

Your landlord may also have to think about the roof. If the roof is soon to be replaced, he may be wanting to not put new units on the old roof.

New units will be 13 SEER minimum efficiency. This is most likely about twice as efficient as your current units. So there will be utility savings. Adding an economizer will also add to your savings by keeping the space buttoned up when you are closed.

Cost of the units and install varies greatly by your area, so I can't really address that. Suggest that about 3 mechanical contractors be called for estimates, unless your landlord has a company they use for all maintenance that is a mechanical contractor. If so, then have them bid, but also get some outside bids as well. Companies that do this all the time have more experienced installers, and better relationships with the distributor for getting the best price, and getting quick help with any troubleshooting.

thanks for your reply.

I believe that "the big giant coil thingie" lol, so technical...is actually just the air handler. Another company come the other day to look at it (sent by the landlord) and thank goodness, they too confirmed that the unit there now is no good, not repairable and would be taken out.

he went out back and looked and said that there are no units on the roof area. So the entire thing will be replaced.

The roof has been replaced within the past few years. I mentioned/asked about an economizer, the guy knew what it was, but didn't really comment one way or the other. I'll be sure to mention it again to whoever is actually doing the work?