If it is a 48x18x24 you have a few options.
If you want full coverage and 0 shadowing when things grow out, go with a 48" fixture. 6 or 8 bulb will work on the tank, 8 bulb for sure if you want to ever get into SPS hard core. Most 8 bulb fixtures are about 47" x 17", so it will cover the entire tank with light.
If you know you are going to stick with a generally mixed reef tank, you could do a 36" fixture. You would just need to account for lower par on the sides of the tank if your rock work extends to the ends of the tank. Really it is more to "Is anything going to be in the way to block the light?"
I prefer to go to the next step up of what I think I will need with lights, as they can always be dimmed (if you go that route with a dimmable fixture), or raised up if needed. It also allows for growth if you decided to get a bigger tank down the road. Use the old light till you have saved up enough for a longer light for example.
As for bulbs, it gets to be more of a preference thing for looks. Myself, I prefer more white and less blue for mixed tanks. I am not trying to make zoas pop like crazy. I want to see the hues in the SPS that are natural. But that is me. For an idea of what bulbs to use to get the look you want, the link and snip below will help with that. And note: you do not have to use ATI, this is just a good reference for color of bulbs and how they mix.
https://www.atinorthamerica.com/t5-bulbs/
Once you decide on a fixture, click the links for the best bulb combo for that bulb amount fixture. It will tell you the mix to get the color you want.
