#1
I play rhythm guitar (punk, metal, metalcore) but want to change the lead guitar (only metal!) But I have no idea what songs (solo) are well suited as a beginner .
#2
Start with simple punk - Ramones, Sex Pistols, Buzzcocks, The Damned and the like. Learn a couple of "easier" songs; Bodies by the Sex Pistols, something like that. Then once you get the hang move on to something a little more challenging.

In the meantime practice a few scales - start with your basic minor and major pentatonics, move up to a blues scale or two and memorise the keys and various shapes up and down the neck. It'll make playing lead and identifying notes in the long run easier.

Maybe even attempt to write your own song and insert a solo. Do it slowly, build up some speed and then even embellish it as you improve in terms of speed and technique.
It's an opinion. It's subjective. And I'm right, anyway.
#3
I'd recommend "Paranoid" by Black Sabbath, it's a fairly easy solo as far as metal songs go. It's in the minor pentatonic scale.