As I understand it,Pete, few differences exist other than cosmetic.
Some Police models have machined parts like extractors instead of MIM ones.
Some letter grade trap and skeet guns have tweaked triggers.
Newer Expresses have a different mag spring and cap retainer setup.
I do know this.....
Over that last half century I've shot 30-40 different 870s, from agency weapons to my own and those of friends. Maintenance ranged from none to obsessive.
Parts failures on all these can be counted on the fingers of one hand, with fingers left over.
Newer 870s have had some QC issues, but the worst is past on that. And the worst 870 I've ever seen was still a good shotgun.
Get the 870 you want and shoot it plenty. Chances are you'll not have any problems.