Wow, quite a few relatively BIG questions in one easy post LOL.
1) Generally controllers that are supported by Serato (and usually come with the Intro version) will have a good mapping for all buttons, faders, jogs and switches. So no need to map anything. Serato does have midi mapping (or assign) functionality, but I have never used any of it. Serato comes with pretty good mappings for supported gear and I like my buttons to do what they say they do.
2) While I don’t know for sure, I believe the Hercules 4MX is not Serato supported. Which pretty much means you are out of luck on that one.
3) There will be a learning curve, but more of a getting used to. DJ software isn’t rocket science and the general features are there in most versions, just implemented differently. Specific features for each software need exploring, learning and practicing of course. At the end of the day it’s your workflow that should dictate what software will work best for you.
On thing that IS pretty significant is switching software in and of itself. If you have a decent collection all set up correctly in your DJ software, with start beats, beatgrids, cue points and loops and such, switching most often means you have to redo all that stuff. It’s why picking software that will most likely be the best for you so you don’t feel the need to switch in the future is important.
I am biased when it comes to the Denon MC6000 (had the original one for a few years since it came out). While I use a Pioneer DDJ-SX now (result of a gear swap), I am still eyeing the MC6000 mk2. Funds are short now, so not happening anytime soon, but high on the wish list. BUT, this is because I mainly do mobile DJ work and the MC6000 is singularly suited for the purpose. It comes at a cost though, it’s 19″ form factor does mean it’s more compact than most others and there is a boat load of stuff tucked into that relatively small space. Consequences are jogwheels that are a bit on the small size and a bit of a cramped feeling, which is particularly nasty when you do a lot of knob turning as some DJs do. Since I don’t (and most mobile DJs can’t be bothered either) this is not an issue for me.
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