If you're doing an entire property, you can make significant interior improvements in your units for $5,000 a door, as that can usually include new cabinets and countertops, new faucets, new lighting fixtures, new flooring and painting (depends on materials selected). If you want to do it a unit at a time, it will cost more, as contractors work best on volume pricing models.
You can do it with residents in place, though you'll need to have them stay elsewhere for 4 nights, as this scope of work can be accomplished in 5 days. Having them out of the unit for the rehab time can be accomplished by giving them an allowance for the inconvenience...don't get involved in making hotel arrangements. The size of the crew will dictate how many units are completed each week, but it's pretty neat to see your property change in a short period of time@
Exterior renovations, including roofing, siding, windows, doors and painting can usually be accomplished in that same time period, though the scope of work and weather are other determining factors.
If you need more info, let me know, as my husband's company has rehabbed thousands of units in this manner.