For the tone & just overall 'amp' sound, the cab & mic are crucial, and wrapping your head around the different combinations of cab/mic isn't easy at all, given the vast number of mics, cabs, placement options, etc. So I always recommend The best IR in the world, really sounds great with pretty much any genre, simplifies a lot the entire tone-finding process. setting the Sag setting of the amp tends to make things tighter; less muddy. Always at 0 for me. Proper use of EQ
For the impedance issue, if you don't have a pod go with the impedance issue then you don't have an impedance issue and adding buffer pedal shouldn't make a significant change.
But that said;
Which particular amp(s)? Not all models nor all amps are created equal. https://www.youtube.com/watch?v=Bz9tO4-kAG4 L6 really models the amps electronic components. And even 2 amps of the same brand/model can sound different, not to mention tubes... The blind tests I've seen of real gear vs L6 Helix/Go/etc., pretty much demonstrate that it's on par; really hard to differentiate. It's really all about your ability to tweak the tone.
And not everybody likes the same amps/tones either. A lot of the amps you've mentioned are favorites in the lineup, and for me they're just plain meh.. Don't think it's particularly related to the amp/model, maybe more about your own playing style, your guitar, preferences, tweaking ability, tone-preference, etc.,
So as there's a ridiculous 100+ amps in the thing, just find a dozen amps that work for you, and forget about the rest. That would be my advice.
But, it's definitely fun to explore amps and their history, and learning how people use them and how to make them work; plenty of tutorial videos and tweaking PGO tones, so I'd definitely start off with that too!
Oh and don't be afraid to use the L6 amps either. Revisited them recently and they're pretty awesome. Definitely nothing wrong using them over 'real' amps; they're just that; improved versions or tweaks of existing amps; amps that could exist, or maybe couldn't due to electronic vs DSP, but honestly they sound great, I'd get rid of all Marshalls in PGO instead of them in a heartbeat! Not 'worse' in any way.