These models all came out at the same time, and as such I'm going to go ahead and assume that they all have the same basic chassis/mechanism. So, to save myself a bit of time (and money), I'm going to review them all here based on the one GP35 I actually purchased.
These are great looking models, and not bad runners at all (at least relative to the era in which they were produced). Pickup is great, slow speed creep is fine, and overall throttle response is excellent. They run a bit louder than more modern models, but not horribly so. Much of the gearing is metal and the multi-piece driveshafts tend to flop around, so I'm assuming therein lies the source of the noise. No problems on narrow radius curves or Code-55 rails.
These models have no traction tires and no lighting. They don't come with couplers, but MT-friendly pockets (and screws) are provided. Ala-carte snowplows are also provided. My factory-painted version came with window inserts (I'm not sure if the undecorated ones are so equipped). The motor is an open-framed 5-pole job, the driveshafts are plastic and pickup is provided by all the wheels. On the downside, the wheel wipers aren't particularly elegant (riding atop of each wheel), and then you have all those wires running all over the place (and distressingly close to the drive shaft). No, these are not "state of the art" by any means. Still and all, great looking models that run very well.
Many of these old Samhongsa diesels were cursed with poor quality plastic gears (particularly the white plastic "compound" gears inside the worm gear towers). Said plastic gears were prone to shrinking and/or cracking over time. However, different suppliers were used for different models and, as a consequence, some models tend to have a lot of gear problems whereas others don't. And at least based on the GP35 I test drove, it would appear that this particular run of models wound up with good gears. However, should you find yourself in need of replacements, NWSL should be able to hook you up. I believe their 1074-6 23 tooth gear is the one you'd want for these models.
To remove the shell, unscrew the two screws on either side of the forward truck. The shell should slide up and off quite easily at that point.
High-Nose and "B" units were also available as part of this release. They rarely show up on eBay, so the production numbers on these must've been pretty low.