If #1 is a 17 series, that means it has only a single wash arm, so you need to be careful about loading the lower rack.  It's built like a tank and will clean just fine when properly loaded.  It was advertised as "A hurricane in a box" when new.  Additionally, since it's 100% Hobart, some parts are still available through Hobart. 

If #2 is a 19 series, it doesn't offer a heated drying cycle.  This is the only series that didn't.  It's also 100% Hobart though, so there's that.

#3 and 5 are the same machine.  I think it's a 21 or 22 series which while based on Hobart's design, were produced after Whirlpool bought KitchenAid.  We had one of them for 18 years.  These only offer a single final rinse, which many here don't like, including me, but I didn't know that when we bought ours new.

#4 also looks to me like a 21 or 22 series, and if it is, then again there's only one final rinse.

While #2 and #4 are more top of the line than #1 and #3/5, they have rapid advance timers that are no longer available and hard to find.  If the timers develop problems, the limited replacement options will often send an otherwise well-performing machine out to the curb.  Make sure that's not what happened with either of them.

I would love machine #2 if it were a 20 series and would even take a chance with the timer if that were the case.  I always use heated drying cycles but if that's not important to you and it's a 19 like Bob has said, then you may want to consider it if you can test it on a full cycle first.  I think my first choice would have to be machine #1.   Machines #3/5 and #4 are non-starters for me due to their single rinses.

Thanks Ben, 

I could sort of make out that verbiage on the KDP-20 but wasn't sure about what it signified.  This makes a huge difference.  I agree with you.  Machine #2 is worth going after, and stainless inserts can still be found for the tub door and the access panel below it.  #s 1 and 4 are good second choices, if you don't mind #4's single rinse.  Same goes for #4 with the stainless inserts, but not for #1.   If you choose #2, take a good look underneath to check if there's been any leaking from the tub that made its way to the motor, and if so, proceed with caution.

I'll also state the obvious:  condition is a huge factor.  If one of these machines is in like new condition, even if it's not your first choice, it should be considered.  Conversely, if any of them have been worked hard, have wobbly wash arms that don't spin smoothly, signs of rust and crust, deteriorated racks, liners, etc., they're probably being pitched for a reason (or two or three) and may require more work and parts locating than is worth your trouble or feasible.