First of all, you should be aware that even if your goal is to be a lead player, you will probably find yourself playing just as much - if not more - rhythm as lead. The reason: rhythm guitar makes up the majority of a song.
Having said that, you don't have to know all that many chords to play good rhythm in most styles.
Do I have to know chords to play lead?
Strictly speaking: no, but it helps.
Unless you are playing unacompanied solos - like Van Halen's Eruption - you will be playing lead over some kind of harmony - that is: chords. What chords you are playing over determines whether the notes you play sound good or horrible. Now, as long as you are just learning to play other people's music, you don't really have to know why it works - you already know that it does. However, if at some point in time you begin to create your own leads, you will have to deal with the fact that certain notes work with certain harmony and others do not. You can, of course, just settle for trial and error, but there are two dangers down that path:
You will have to discover what works and what doesn't on your own, so be
prepared for a lot of groping around in the dark,
When you discover several ideas that do work, you might find yourself using them all over the place, which runs the risk of having all your leads and solos sounding the same (and thus boring).
Picking up a general knowledge of chords as you go along provides a safeguard against both of these. It will increase your understanding of what you are playing - even if you are just learning other people's songs - and that makes it easier for you to come up with your own ideas that work when you wish to do your own thing.
Is it not enough to practice scales alone?
I would suggest that practicing scales is not that important, overall, even for lead playing.
Why do we practice scales at all? Firstly, to achieve a measure of technical proficiency with single-note melodic lines. Second, to learn the note sequence and basic fingering patterns for a given key. These are important skills, but not in themselves sufficient to create interesting and exciting melodies and leads. Think of it this way: when you learn to write at school you will be practicing your handwriting and spelling just to be able to write something that will be legible to someone else, but a mastery of these two skills is not enough to write an essay or a story. So it is with scales.
In short: practicing scales is only a small part of developing lead technique and style and by no means the most important. I'd go as far as saying you can develop good lead skills without practicing any scales (although I would not suggest this).
How well should I know chords?
You know those chord dictionaries with the weird fingering diagrams and names like F#7b9? Depending on the style you are playing, you can probably ignore 90% of that.
Guitar George may know all the chords, but you probably will only need a small fraction of them and most of the chord shapes you will need will be movable - that is: you'll be using a single fingering which gives you the same type of chord with a different root (C major, D major, E major; A minor, B minor, C minor) depending on which fret you start with. Most of the chords in the dictionary are the same fingerings played in different positions of the fingerboard (the same goes for scales).
In the long run, it is much more important to know at least the basics of chord construction (which notes make up a particular chord) as well as chord relationships (which chords go well together and when) than memorising all the various fingerings.
You will probably want to learn at least the basic open position chords at some point (chords played near the headstock, using open strings), the movable power chord fingerings (since your question suggests you'll be wanting to play metal and rock stuff) and maybe some movable barre chord shapes to complement that.
What should I practice every day?
If you are already following a guitar course, you should stick to it at first and make sure you are really working on all the exercises and drills that it gives you. Every course follows some method and the later, more advanced material assumes you are already comfortable with the skills presented earlier.
What about practice outside the course? Since you are just starting out and don't really have a lot of time to practice, you will probably do best by learning to play some actual music first, before diving into more technical things.
Find some riffs, simple leads and songs that you can begin learning. Take some time to master one and then move on to the next. This will help you stay motivated because you will be playing music, as opposed to going through drills, and it will help you build up a base for more advanced concepts.
The single most important thing: Get a metronome, drum machine or computer equivalent and learn to play along with it. This also means learning note values and rhythm notation, as well as counting while playing.
The metronome is an indispensable tool when learning to play. It will prepare you for playing with other musicians, teach you to play in time - which is an absolute must, whatever it is you are playing - and allows you to pick apart difficult passages by counting them out and playing slowly at first and then gradually increasing the tempo.
Ideally, as you are learning new music you should be looking at what you are playing and thinking about how it relates to other things you already know. You might start noticing patterns about how the music you are playing is put together and see them repeat in different places. Take note of it. If you are following a course, notice how things you have learned show up in the music you are learning on your own.
The goal here is to have an understanding of the music you are playing. With time you should not only be able to play more complex material, but also have an idea of how the elements of the music you play fit together.
Bottom Line
Every guitar player needs solid rhythm skills - even if they are primarily a lead player - because most songs are mostly rhythm accompaniament and the notes in a lead will need to be played with good timining.
Timing is the single most important skill you should be developing when learning to play.
Knowledge of chord fingerings and scale shapes is a lot less important than knowing how the notes in chords and scales work together musically. It is a good idea to gradually gain at least some understanding of how the music you play works - especially if you ever wish to try a hand at writing your own - but, luckily, it is a skill set that can be developed along the way.
