For lighting it might be worth checking out this guide, it's a little out-dated -
This has some stuff you might be interested in -
and this isn't for quake but you may find the general advice to be sound -
My own advice when lighting is concerned -
If you have a light (like a spotlight) then put a bright light near the source, like something that is light 300-400, but make the "wait" key 3 or 4. Then use a fill light. That is a light that is lower brightness, like 100-200, then use "wait" of a sub-decimal. Maybe 0.7 perhaps.
Doing the above gives you a nice bright light source but fills the room with a nice level of ambient light. You could even double up on the spotlight effect by having a bright light near the ground where it would hit.
You could try messing with the "delay" key, mappers like Sock tend to create fill lighting with a delay of 5, a wait of 2 and a light of 1000. This will ensure the light will fill an area nicely but wont look super bright due to the way delay works.
Another thing with lighting is shadows. Having the interplay of light and shadow is always good. Think about how you can obstruct a light in interesting ways to cast shadows with interesting shapes. It's not practical in a realism sense but Quake is a very abstract world.
Lighting takes practice, and I would say copying is the best way to achieve your goals. Try and emulate nice lighting scenes, look at other maps that have good lighting and emulate it. The key to making maps and getting good at it is making test maps. I guarantee that every mapper here has 5 times as many test maps than the amount they release.
When balancing weapons and powerups I find you need to ensure that a player has to work hard to dominate. So make weapons spread out as much as possible. Some weapons you will need multiples of, like the super shotty and nail gun. This enables new spawn players to be able to arm up against players with the rocket launcher or lightning gun. Put powerful items in more dangerous places, this gives you a risk/reward scenario where taking risks net you a reward. Like putting a Quad in a tricky jump over lava, or a rocket launcher at the bottom of a central atrium (moving into an open area makes you a sitting duck but if you get the gun then you'll be powerful).
Put powerups in secret out-of-the-way areas.
I used to love making DM maps, getting the gameplay balance right takes a lot of thought.