What should I start practicing first?

Hello everyone,

I am 57 years old and have been playing rhythm guitar for several years. I know open and barre chords, circle of fifths and fourths, chromatic scale and that's all. I bought an electric guitar and I would like to learn the phrases and solo parts of my favorite songs such as Sultans of Swing, Money for Nothing, Back in Black, Highway to Hell, Stairway to Heaven, The Wall....I know that it is not a good way to learn specific solo parts without having the basics of alternate picking, legato, arpeggios, scales and what else is needed to become a solid lead guitarist.

I lost myself among the hundreds of tutorials on YT and don't know where to start anymore. I can practice for about an hour and a half every day. I hope someone can help me with which exercises to start, to guide me so that I don't wander. If anyone wants to do it at all. Thanks in advance.

Personally I don't think you need to have the basics...

{of stuff like "...alternate picking, legato, arpeggios, scales and what else is needed to become a solid lead guitarist..." } 

...down BEFORE you start learning to play your favorite songs.

Many people use the parts of songs to develop those skills.

However...
What I think you - and everyone; who wants to play guitar - needs is:

{a fundamental understanding of how to make music with guitar}

Lots of people learn scales and chords and etc... but they can't actually make music; because they don't understand all the flashy licks and solos in context of the fundamentals.

so my advice

1: commit to a course that takes you through the fundamentals

2: learn the songs you want to play
(free tempo) 
Don't try and play that stuff seriously until you can play all the exercises in the fundamentals course PERFECTLY with ease.

3: Start playing with other people ASAP

Anders is a working professional musician
Which is why I think Guitar Tricks Fundamentals 1 & 2 will give you good foundation.
He knows what you REALLY need to know and do in order to be a competent musician.