Seems there's a lot of keyboard commando hatred for the Express guns out there on the Intarw3bz for some reason. I can't figure why, I have owned a dozen or so of them, all bought used, and the only problem I ever had was with a used Police barrel I put on one, once upon a time.
Several years ago Remington changed the design of the magazine spring retainer and magazine cap detent on the 870 Express. Rather than the old style spring steel cup-shaped insert to retain the mag spring, and a spring-loaded detent set in the barrel ring to retain the magazine cap, they went to a molded plastic part inserted in the end of the mag tube to do both jobs. That plastic part is retained by two 'dimples' pressed into the magazine tube near the threads at the end. Those dimples get in the way of installing magazine extensions, and are a pain in that regard. I quit bothering with newer 4-round Expresses that have mag tube dimples - just too much trouble.
The only problem I know of with the newer version 5077 is getting a sporting barrel to fit it. With an older version of the 5077, the one with the factory magazine extension, that won't be a problem - there are LOTS of sporting barrels around for that version of the 870. All you will need to do is get a standard 4-round mag spring and an old style magazine cap plus a magazine plug if you intend to hunt migratory birds, and you'll be in business.
For a dedicated defensive shotgun, the new model 5077 with the one piece extended magazine tube is a great starting point IMHO. I look forward to finding a good used one to start playing with and maybe working on a bit down the road ...