When I run a multi-colored deck, there are 3 major things I think about:
1. Can I reliably play my cards on when I want to: This sort of combines two smaller things, namely, do you have enough sources of each color needed, and are too many lands coming into play tapped when you need them untapped. For example, if you're playing a deck with a high mana curve that really turns on at 4 land, then the Fastlands (like
Inspiring Vantage,
Blooming Marsh, and so forth) are great for getting you your colors, but are going to be coming in tapped when you need them. For that kind of deck, the checklands (like
Glacial Fortress,
Drowned Catacombs and so forth) are going to be better since by turn 4 you're more likely to have a basic (or shockland) in play. If you decide to go with esper colors, and run a lot of the cards suggested, you'll want an untapped black land turn 1 for a discard spell,
The Rack, or
Shrieking Affliction, but after that, most of you land can come into play tapped without hurting you too much.
2. Am I completely two+ colors, or am I just splashing? If you're playing a black deck that also runs Tidehollow Skuller and
Castigate, that's just a splash, as most of the time, black mana is more important that white. When splashing, it's much easier to
balance the mana base and you pretty much just basics in your primary color, and your non-basics reach into your splash color. This way you'll never be short of your primary color (in your case, probably black). Splashing is also nice because it's much cheaper $$ wise (since you can run a lot more basics and probably don't even need fetches and shocks.
3. Is the Extra color or Splash worth it? There is tradeoff when going into extra colors. Without a solid mana base to support multiple colors, missing one of your colors when you need it is a real possibility. You have to ask yourself if the value you get from your other colors (cards like Tidehollow Skuller and
Castigate) are worth the risk of not being able to play them (or worth the lifeloss from the lands you'd need to play them). 2 colors are usually pretty easy to pull off, but when you get up to 3, it can be a real challenge requiring expensive cards to make sure you regularly hi tall your colors.
In the end, if budget is a factor, and you just want to play a fun / powerful discard based deck, mono-black 8-rack is probably the best option, with maybe a white splash for removal and sideboard cards. If you wanted an idea what this could look like, feel free to ask