The units are about 500 sqft each, 1 bed, 1 bath, kitchen, living room and 2 closets.

Currently kitchen, laundry and downstairs bathrooms have linoleum floors, except master bath and walk in closet has carpet. 

Cost and portability are not important.  

And it looks portable too. 

Each unit has a storage closet under the porch that comes with them. 

In my area it's steep driveways, tuck under garages, small bedrooms and closets, 1 bath (and small), tight kitchens, steep stairs, 4 level splits, train tracks, etc

I looked at a used stackable W/D set, 24" wide model, which will fit just right in the laundry closet, for $400 with delivery, or $525 with 1 year warranty.

These bedrooms have enormous master closets and attached master bathrooms.

I want to say all in, including the trim, new doors to the room and closet I spent under $1000.

Unless you are able to open up the closet space where the water heater currently is (assuming it is indoors) and use it for more living or storage space creating value of usable interior space, then I don't see why you would want to spend more money on tankless as opposed to the less expensive tank heater.