You have a few different options, any time you require a compact washer and dryer for your rental units in your apartment complex. Deciding which ones would work best in your units depends upon the amount of space you have available. If your units are older, the buildings may not have any washer or dryer connections. Rehabilitative and remodeling costs for adding these connections could become quite expensive, especially if you need to run dryer vents. However, there are solutions available to help keep your project costs under control and within your budget.
Your first option is to consider a stackable compact washer and dryer. Each appliance is a separate unit that can be installed by placing the dryer on top of the washer, or setting them side-by-side. The dryer is ventless, so there is no need to run a dryer vent. The only connections required are a hot and cold water hook up, and a drain for the discharge water. The only construction cost consideration for your project is the expense of adding the washer hook up.
Your other option is to look at an all-in-one combination washer and ventless dryer. This is a single appliance which gives you a little more flexibility in its placement in your units. You could install the washer hook ups in a fixed location, or add a wheel kit to the unit, so it could be rolled up to a kitchen sink and connected directly to the faucet with a portable conversion kit. The hot and cold water is supplied to the washer from the faucet, and water discharge goes right down the sink drain. Your project construction cost considerations with this model would depend upon whether you wanted a portable solution, or a permanently installed solution.