Just about every day I'll get an email from a random A Cappella person asking if he or she should buy a particular mic, preamp, plug-in, monitor, headphone, recording platform, sound absorber, and so on and so forth. I spend a lot of time responding with a very vague, individual long answer, but this really is a "one-size-fits-all" situation, so I'll just refer people to this article from now on!
The answer is "there is no answer... at least one that I can just email to you". I know it's frustrating to be told that you need to find what works best for you by yourself, and the reason I know is that I was in the same place 30 years ago being told the same thing. It's taken me decades to actually understand... well, kind of understand.
The thing is, what works for one person might not work for another. Let's use an example of probably the two best known Vocal Percussionists in Contemporary A Cappella - Wes Carroll and Andrew Chaikin (AKA Kid Beyond):
Wes uses a RØDE NT-3 mic, whereas Andrew uses a Shure SM58. After going through many mics, this is what each found works best with their individual styles. If they trade mics, they both have to alter their tones drastically, neither feels nearly as comfortable, and overall sonic quality can go out the window!
Consequently, one preamp might respond differently when paired with a certain mic than another. So just asking "what's a good preamp" really doesn't have an answer unless you've heard it in your own signal chain.
Then, there's the space you're recording in - different mics will respond differently to that as well, regardless of price or pedigree. Going the opposite direction, different monitors will also be affected by your room in different ways.
Asking someone like me what will work for you is like asking someone who drives one sort of car what they think of another car they've never driven - and even within that there are different styles: "What do you think of the handling on the new Ferrari?" "Uh, I don't know, I drive 4x4 trucks, I'm more into traction myself".
This isn't to say that there is NO answer, but at some point you need to just jump into the pool so you can learn to swim. Learning what DOESN'T work for you is just as important as learning what does. I went and bought some very expensive, world-class monitors that are well known for making hit records, and I couldn't get a decent mix out of them to save my life. I brought them back, and tried something else. Over the years I went through about 10 pairs of monitors before I found what really works for me. "But I want to be perfect NOW!" you say... sorry, I can't help you there, we all have to pay our dues and go through the learning curve of our own situations.
Of course, budgets are a constraint, and you want to make informed decisions - by all means keep an ear open for recommendations, read reviews, etc, but ultimately just using the stuff will tell you if it's right for you. So how do you do this on a budget? Check eBay! I myself have bought very few pieces of equipment brand new (in fact the vintage stuff is the most desirable anyway) and I've never had a problem with used equipment. Buying something used means someone else took the depreciation already, so if it turns out to not fit your (or your group's) personal style, you can just sell it again for around what you paid for it (and if you invest in higher-end gear, a lot of the time that can actually INCREASE in value so you can sell it for more than you bought it for). Also, since you are not paying full retail price anymore, you might be able to afford two or three mics/preamps instead of one, learn which work better for you (accelerating the learning curve), and sell the ones you don't tend to use.
Lastly, once you have a basic system going (i.e. you're swimming in the pool), try to borrow, rent, or trade mics, preamps, headphones, and monitors with others so you can get experience with other pieces of the audio chain. When you find that magic combination that works for you, you'll know!