Buying new headphones + mic combo

Spend a little more and get the HD 558, it's much better than the 518 and cost less than the M50x.
I wouldn't pick neither of these if you're a basshead. If you want something with a little more bass then look into the Philips Fidelio X1.

The 558 is not much better than the 518, they are nearly the same. They both use the exact same drivers, the only difference is in the padding and housing.

The fidelio x1 is surprisingly bass capable for an open back headphone, but will not have the same bass quantity as a closed back pair such as the m50x.

I have owned the m50s which sound nearly identical to the m50x and they would work, but are less viable of an option given other headphones in that price range. Specifically the Beyerdynamic DT 770 PRO.

The Senns have a neutral-warm back sound signature. They are decent all rounders and respond well with an eq, but for those genres I would prefer something a little more fun and exciting sounding. That's where the DT 770 PROs come in. V shaped sound signature (emphasis on bass and treble). Deep and powerful low end, slightly recessed midrange, and fun sparkly highs. The sound is non fatiguing and the comfort is excellent. Tank like build quality as well. It doesn't come with a removable cable however. You might need an amplifier depending on the version you buy along with the devices you plan to use it with.

It's also worth to note that an open back pair will leak a lot of sound and won't isolate as well as a closed back.

I can make further recommendations if you can state your budget for the headphones.

For the mic you could look into a cheap flex mic with extension cables for ~$15, or if you want higher quality the antlion modmic is a popular choice for $50.

Not to my ears. The difference between the 518 and 558 is like 30fps and 60fps. Some people can tell some can't.

Thanks, I do enjoy my time spent on the forums haha.

For £100 I would definitely look into the Beyerdynamic DT 770/990 PRO since uk pricing for them is great. I described the 770 PRO above, the 990 PRO is similar but more refined on the whole since its open back. Larger sound stage, greater clarity/detail, better imaging, with less bass quantity. They would be better technically speaking for gaming since they are not as bass heavy and positional queues would be easier to identify. They do have a treble spike, meaning that high notes are greatly accentuated. As a result the sound is more fatiguing. Some may enjoy this, others might not since they have sensitive ears.

If going for the DT 770 PRO go with the 80 ohm version since it is easier to drive. Both headphones will benefit from an amp, but you likely can get by with your onboard audio assuming you have a gaming desktop.

You need a good amp to drive the K612 PRO. For $100 USD there is the objective 2 (O2), or Schiit Magni/ Magni2. Not sure of UK pricing or availability for those two. If it's not in your budget I would look into the Beyers.

Personally I would think the DT 770/990 PRO are a better choice for your taste in music opposed to the flat sounding AKG.

I would personally get the Plantronics Audio 300 for a soundcard myself.