It will do a better than average job.

But, 18" ers have a built in disadvantage because their tub is rectangular in shape. This means that the spray arm(s) don't cover a larger portion of the corners compared to a 24" version which has a square shaped tub.

Some dishwashers have the ability to do a half load. The downside is you have to load just the bottom rack. When I was looking I thought Bosch also had one.

Have you thought about dish drawer type washers?

Haven't considered them. How do they compare in cleaning and price?

Google dish drawers to see who makes the various models and prices. You should probably start a new thread asking about the drawers that are of interest. If each unit is considered separate, then I would also assume (if you stack) that you will need dual drains, electrical, and water supplies. I have no experience with actually using one.

the Fisher Paykel drawer(s) clean as well or better than average, priced comparable to the bosch 18" models.