Ted, I have heard it said on several occasions that the 18000, the 1989 remake of the 227 family of switchers, is the best modern locomotive Lionel ever made. I do not have one, so it is hard for me to compare the operation. I do know the 18000 was made with plastic gears. My preference for the worm and the worm wheel would be metal gears with proper lubrication. The 001E-185 e-unit used in the 227 family is far easier to maintain than the 259E e-unit. However, if the 259E e-unit is working properly, there is no difference in their operating performance. The motor in the 227 family is the same motor that is used in the 700E and the 763 with the exception of a slight difference in the mounting. In my opinion the Pullmor motor, like used in the 18000, is a better motor than the 700E motor. But since the 700E motor only drives spur gears, the poor thrust arrangement in the 700E motor is not a big issue. I have never seen a 227 family switcher cog, but they are all well broken in, the newest being 76 years old, so it would be hard to compare.
The 227, 228, 323, and 233 all have complicated electrical equipment in the tender for the teledyne couplers or the magic electrol reversing. I do not use these features when I am running and do not believe the extra cost for engines with these tenders is justified. If you look for a 230, with the 2230T tender you should be able to save quite a bit of money. I believe the engines sold on eBay are over priced. You should watch for these locos at train shows and auctions. The 227 family locos that sell at a Stout Auction generally sell for less than what is being asked on eBay.
When the grandkids come over to run the trains, about the only locos they run are the 227s. They run smooth and slower than newer locomotives. However, I do not run any newly purchased engines until I have done a frame up reconditioning. Everything is cleaned, repaired, and properly lubricated, so they should run well. But from what I have heard, if you can find an 18000 at a good price, you may find you are happy with it.