converting kitchen into a bathroom

putting in a toilet without having the right drain pipe will not happen. laundry room is more like it.

Avoids needing to stain the floor.

It all depends on the building. Can you get a drain pipe into the floor? Is there a sufficient large drain stack behind the kitchen? Is there a path for the drain pipe to the stack?

Has anybody else in your building done the same?

Correct about the drain pipe/waste line for the toilet---may have to raise the floor to achieve that which is not ideal aesthetically. Only way to really guess is to get floorplans for both your unit and the adjacent and ask a contractor/architect to have a quick look before you invite them to look at both spaces. It may not be that big of a deal but you would also need to run everything by the co-op/condo board and managing agent of course. Another good idea is to see if anyone else has done it in the building, particularly in your line on another floor and then to ask that owner about their experience. I have seen many people achieve this scenario so it is in the realm of possibility. Everyone's definition of "easy and expensive" is different.

Tsetsefly,

the answer might be using a wall hung toilet. Without seeing it giving a price for that would be meaningless