Could use your advice:
I'm sorta thinking about getting back into model trains (as an adult). I had no idea about the vastness of the hobby. After looking at a lot of train stuff I'm intrigued by these highly detailed brass steam engines.
Is this level of detail available in say a Bachman Spectrum for example or unique to brass?
That is not an easy question. The level of detail of 1960s era brass has been far exceeded by plastic/die cast models today. So there is a continual improvement in the detail through the years. I sold all of my brass from that era and "upgraded" to the plastic models that started coming out from companies like Broadway Limited and Proto-Heritage. Many brass models today are - in a word exquisite -. However when I consider spending the $3000 for a brass W&R model and then think I can own about 8 plastics for the same price the value proposition comes into play and I end up with the plastic ones. They still look as good or better than the brass of old, but I don't get nearly as bent out of shape when one gets wrecked during an operating session.
Not all are unpainted. There are many factory painted brass models. e.g The PFM "crowns" from the late 1970s through early 1980s came painted from the factory. I believe originally they were unpainted because they were already so pricey, they didn't want to add the additional $100 just for paint. One could buy the model now and paint it later. It used to be a huge industry to do "custom brass painting". More and more modern offerings are coming painted. The $100 for paint isn't as big a deal in todays dollars as it was back in the day. All my recently purchased brass has been factory painted.
Would I need to get inside them to update to modern motors etc.
Depends on how old the model is. I believe all current brass releases are coming with can, coreless, or at least 5 pole skewed armature motors. Anything before 1978 probably has old open frame motors that would have to be redone. How hard to disassemble - depends on the model.
I'd want to go Great Northern steam as this is what I saw almost daily as a kid going to school.
Great Northern favored the Belpaire fire box, as much weight on drivers as possible, and they built many of their own locos rather than just purchasing them from Baldwin, Alco, or Lima. As such, their locomotives were for the most part very unique, and many are only available in brass. In this regard brass is not a luxury it is the only show in town. This is the reason I stopped modeling the GN and switch to a road with more standard type locos. There are a few GN models out there in die cast or plastic. They had some USRA type locos (done by BLI recently), they had a couple Z6s (Northern Pacific style which were just produced by Athearn), umm umm a couple others that I can't think of off the top of my head right now.
A couple years ago BLI did a gorgeous brass model of the S2 Northern. I really wanted one of those, but once again couldn't justify the price for a RR I'm not "officially" modeling anymore.