I'll second the vote on the 0-6-0 docksider being an excellent engine. They run great, smooth, pull a crazy amount of cars compared to many larger engines and especially for their size. They are all metal, well detailed, light in the cab so you can see the figures as it runs around.
Only complaint would be that the smoke output is anemic, at least on mine. I have not tried the suggestions on this forum for improving smoke output (new batting, different wire wound resistor, etc) other than switching to Megasteam.
I found my son was in the habit of running the engine about 6 hours a day while I was at work during the Christmas season for the past two Christmas' pulling as many cars as he could - up to having the last engine almost pushing the last car around a 7 x 10 loop with 048 curves. This was without proper maintenance as I didn't know it was happening. Rollers and wheels show wear but the engine runs like new.
Some of these with the "Series 1" motor seem to get dry bushings causing them to squeal and stall. As the docksiders are all quite a few years old at this point, I'd lube the motor bushings as well as the normal maintenance points if you get one. If the series 1 motor goes belly up, Lionel is out of them and has no plans to get more. You can get the original, or an upgraded motor from a repair depot.
