That price should include the inside unit to be replaced too. Your not able to just change out the outdoor unit as they dont make the older R22 unit anymore so you have to get a new inside and outside until that runs on 410 A. The price sounds okay we pay $3800 for a new install of a 2.5 ton but we do a lot a business with this vendor. You can try to shop around you maybe able to get a bit lower. The cage idea would work. I would wait until you have it rented and arrange to have installed the day of the move in with a cage. And whenever they do move out in between your next move in, keep lawn short, power on inside with an exterior light on but the easier is to try and pre rent it and have only a couple days between move outs. Window units are okay, you will have to decide what you what to do about heat because most window units only have cooling and if you want heat it will need the 22) volt plug/wires in wall. Window units can also attract lower end tenants and you may have a harder time getting a good tenant. They can get stolen just as easy and if you have a bad tenant they will or could steal those window units very easily too when they move. I would go with new central air unit install day of tenant moving in and pay extra for the cage and shop around the price to get to $4000k if you can. Also tenants may complain after they get there electric bill window units cost too much and if they see the vents on the floor and the rough in for the central it may be an ongoing nag from them to install.