The best investment any studio can make is in a good set of studio monitors. This holds true for putting together your first home studio on a budget, and for professional working studios alike. People often say "you can't fix what you can't hear", implying that if your monitors do not display a problem, you won't be able to fix it, and that problem might surface later on when the mix is being played back on a different system, such as a car stereo (resulting in many engineers running to and from their car for the fabled 'car test').
Selecting monitors can be daunting, especially for new engineers. It is difficult to know what to listen for, and buying based on the recommendation of a friend or online reviews is often the way people will end up going. Even for more experienced engineers taking the step up to a higher echelon of monitoring, it can be difficult to tell.
There are really 3 things that I like looking for when it comes to selecting monitors at any particular price range:
Do you like listening to them? There is no sense in working on speakers for days and hours if you feel like they are shredding your ears, or if you don't like listening to music on them. Music is about feeling.
How accurate are they? Specifically, how well does the amp control the speakers so that they can display all of the subtle details of the audio.
Are there phase issues? Has the monitor been designed in such a way that the crossover points are not going to be problem areas? I would say this point is not particularly relevant for introductory level monitors, but becomes increasingly something to pay attention to as you are moving up your price point.
Notably, I don't think frequency response is hugely important. This is the main feature I think a lot of people are looking for when they are selecting monitors. How 'flat' are they? What is the bass extension?
All studio monitors are designed to be 'flat'. That is to say, if you run a sine wave through every frequency, there should be as little variation in how loud the speakers play back any particular frequency. I have spent a good amount of time listening to a lot of high end monitors from Dynaudio, Amphion, Focal, PMC, Genelec, and other 'high end' brands. What I've noticed is that all of the monitors have tendencies to focus on particular frequencies, which would suggest likely intentional variations from a truly flat response.
With the amount of time spent in designing these monitors, it is unlikely that PMC, for example, wouldn't be aware that they likely have a slightly increased response in the treble frequencies. So are they bad speakers? Absolutely not. They are fantastic. But that is because they are accurate, as described in my second point, and there are no major phase issues in any of their lineup, at least so far as I've heard. Then the only point left is whether or not you like listening to them. Personally, they didn't work for me, but they worked for the engineers who owned them.
As an aside and related, I find a lot of monitors that are 'brighter' are perceived as more accurate or detailed, and therefore better. A lot of how humans perceive sound is dictated by the high frequencies. The highs are where a lot of 'feeling' comes from. But just having these frequencies more out front just means they are easier to hear. You can still hear them on other equivalently accurate monitors, they just aren't accentuated to be the first thing you hear. There are pros and cons to this, but don't buy the brighter monitors because they are 'more detailed' unless you like them better and actually hear things you can't on other monitors!
The other big part of why 'flatness' doesn't matter is the room the monitors are set up in. I currently am using a pair of Focal Solo6Be's. I am a very big fan for the reasons that I've listed as important. They are not a perfectly flat monitor. They weight their mid frequencies slightly down, in what feels relaxed relative to many other speakers. That being said, I am very confident with them and they are very intuitive to how I like to hear music. I heard a set of the Solo6's in a show room a few weeks ago, however, and they were a completely different beast. The bass response felt odd. The highs almost felt shrill. Nobody mixes produces in a room that would enable a flat monitor to translate truly flat to the listener, so why design a truly flat monitor? No matter how flat a monitor you design, it will never sound flat in practice, as the room and acoustics of the room play too much of an impact.
Monitors are designed for people to listen to them. Everyone hears music differently. Everyone will use the monitors in different space. Just because someone else likes a set of monitors, does not mean they will work for you. For example, I can't stand Yamaha NS10s. There are many speakers that have been produced since them that meet or exceed their accuracy (which is also related to the amp in this case), phase coherence, and frankly that I think are easier to listen to. A lot of why I think they are still so popular is because people are just used to mixing on them. Which is fine, but if you are new to studio monitors, don't assume you have to get them!
The bottom line I think is to just listen. There are so many fantastic monitor companies out there at every price point. Every company will have a different flavour, so try out any that you can to figure out what works for you. You will enjoy recording, mixing, or mastering a lot more if you like what you hear!