I just installed the Home Depot luxury vinyl planks in the kitchen and living room of a rental along with tile in the bathrooms and both look great.

If kitchen cabinets, counters, bathtubs, showers etc are in good shape, there's no need to replace them.

Gas will require a room with fire suppression included.Size and finish of the kitchen and appliances. 2, 3, 3-1/2, 4 bathrooms?

Example If the kitchen cabinets are 20 years old, they will find the value of 20 yr old cabinets and pay you that amount.

I am trying to figure out how a gut rehab of an apartment can only cost $2,500 to $3,000, including a washer and dryer and Refrigerator, and stove, and dishwasher, and kitchen cabinets, and counter tops and tile floors and bathroom fixtures, and painting, and electrical, and plumbing.  

The new floors will abut existing tile floors in the kitchen.

I want to add a small kitchen to it and rent both as separate units.

In my opinion its better you consult a kitchen renovation service or company , and clear your doubts about it .

I would highly recommend it especially when the things that have the greatest return are bathrooms and kitchens of any property. 

New kitchen, new bathrooms, new flooring, the works.