Hi Matt,

Welcome to the forum. I have bought second-hand, 3-pole locos in the past (but not models of steam engines) and they have been good runners (they have been well maintained). However, I believe it can be a bit hit and miss. I think that slow running is a combination of having a well maintained loco, clean track and the type of controller you are using. The older Marklin mini-club controllers were not particularly sensitive.

Carim

I only got into Z scale (and trains) recently. We are in the same boat. Good luck hunting but I found that the prices of the various locos doesn't change much going from the older 3 pole to the newer 5 pole to the newest "bell shaped armature" (I think that means coreless) motors. I don't have any 3 pole motors and I wouldn't avoid them but I wouldn't buy a 3 pole engine and then spend $45 for a 5 pole replacement motor just to upgrade it. I also think any reputable seller would say on the ad what the pole count is. I am not sure about the site rules but I started with Z scale hobo and he is very good about explaining the drive system, even including videos of the engines in action sometimes. So you can learn a lot just surfing that site.
I have had mixed results buying older engines. These little locos just require attention and a bit of small scale skills to get or keep them running smoothly. Just because the ad says N.I.B. doesn't mean it's going to run like a champ. If it has been in that box for 20 years or was tested briefly and put back in that box because it wouldn't "go" then you will have to deal with it. Lucky for me I have really enjoyed working on my Marklin trains.
One great advantage of the newer engines is the LED lighting, which turns out to be a big deal in my book. I have gone back and replaced the bulbs in a couple of my older engines with the LED replacements from High Tech Modelbau and it is a nice improvement. I have also gotten older Marklin "anniversary set" engines that do not have lights at all and I was disappointed about that.
Well as you can see I really like your post as it hit close to home for me. Marklin keeps bringing out updated versions of their older locos, so if you have an engine you crave you can probably find it with any complement of motors and lights, although the bell shaped motors are just coming out recently. Maybe try the new #88741 or #88742 engine and compare to what you have?
Happy hunting.