First off, what's your technical skills in diagnosing and repairing electrical/mechanical failures? What's your desire to learn and tinker? Are you prepared to seek out obsolete replacement parts and store them before they fail such as timers and program switches?

The 18-20 series machines are 37-47 years old now and while they were rock solid reliable back in the day, they're going to require some maintenance before being put into regular service. Too many here get caught up in the hoopla of owning a vintage machine but have no skills to diag or repair with absolutely no desire to do so and the machine soon gets trashed when a problem arises or poor performance occurs.

Expect worn/rusty racks. These are getting hard to find and are quite expensive when found on ebay. Expect to pay north of $100 for decent racks and that doesn't include shipping. Expect to regularly touch the racks up with the proper paint and add rubber tine tips along the way.

Expect worn upper wash arm support and worn wash arm bearing. The upper wash arm support and wash arm are NLA.

Expect to replace the water valve. Hard water plugs them up and the machine underfills. The original ones eventually leak. You take a risk at destroying your floor if it isn't replaced.

Expect to replace the lower seal seal and drain impeller if you don't want to purchase a new motor in the future when the seal fails and water takes out the motor bearings and your kitchen floor. I've never been inside the KA that didn't have glass in the sump and at least a worn/chipped/destroyed drain impeller.

Expect leaks around the hinges of the soap dispenser doors. They can be fixed with new O-rings...but expect to repair it.

The 18's have an accordion seal for the upper rack that's impossible to find these days. You're up a creek if this fails, which most have by now.

Expect to replace the water delivery tube in the upper rack. These deteriorate overtime and make the entire systems lose pressure.

Expect the timer and program switches to eventually go wonky and require replacement....remember these are 40+ years old now. They have been NLA for decades now but still occasionally pop up on Ebay. There's an NOS KDS-18 timer on Ebay right now that on Shoppers Square on the eBay watch that I posted a few days ago. Nobody has picked up on that yet.....

Expect to purchase old school Cascade Boilout detergent if you want decent results, not available in stores...you must order online. Pods and tabs will sudslock these machines in less than 2 minutes and that's with hard water. Liquid detergent leaks out of the dispensers quickly...these machines were designed for powdered detergent only. Modern enzyme detergent doesn't have enough time to work its magic in the quick cycles these machine have.

As to what machine is the best that depends on your situation and the upgrades you're willing to perform. The higher end 18's are the only one of the 18-20 series that has true 3 level washing. They are the last series to have a 180F sani rinse. They also fill up with more water than the 19 or 20 and have more aggressive washing action. The wash arm in the upper rack is smaller and doesn't get the corners as well. You can slide in a 19/20 upper rack without modifications. The 18 does not pre heat the water meaning you need 150F out of the kitchen sink tap if you want good results. The 19 and 20 will preheat the water to 150F and allow for lower water heater temps. Most people set their water heater at 120F and it's usually around 100F at the kitchen sink tap. You will get crappy results at those temps.

Still want a vintage dishwasher?

The 15, 16, 17 washing system was the Hydro-Sweep arm however, the 15 had a weird upper rack, in my opinion.  The fold down sections with the loops can be a bit challenging to load.  And the cycle selection options for the 15 were fewer.  But don't get me wrong - my 15 still gets put on KP duty in the Ogden kitchen from time to time!  Compared to a 14, the capacity is astonishing.