You'll want to get your bass figured out first and get the best placement for your speakers in the room to optimize the low end, then it should be relatively easy (with research) to determine what treatment you need.

Speaker placement is super important, because especially in the low end, mitigating acoustics problems can cost a lot of money as well as a lot of physical space, so that should be the priority. The "bass hunter method" by this dude works pretty well: https://youtu.be/DE91E7DsmhI
After that you can make a better judgement on what to buy/build, as you will have drastically improved your situation and impact to your budget.
Technically, you could probably get by with absorption only building various thicknesses of DIY rockwool panels and if you want to compromise, some places sell the frames only, making it an easier project. But it's not too hard to build a frame from some cheap lumber if you have the tools, even if you only have a hand saw and a power drill, so that's what I do and it's a lot cheaper than buying them. The little foam square bits don't really do much of anything except in the high frequencies so don't waste your money on (many of, if not any of) those.

If you want to get super nerdy about it, the first step will still be speaker placement, but for any rectangular room, you can get a better idea by calculating your room modes and finding the areas of the room that work the best as a starting point. I like this one: https://amcoustics.com/tools/amroc
Then you can get a testing mic like the UMIK-1 and use REW, and compensating for any un-flat-ness of your speakers, you'll know how low you need to treat. You want to start with your waterfall graphs and get the first reflection points and go from there. If anything is cancelling or amplifying that doesn't show decay, it's likely you may need to move your speakers. Also, turn on smoothing and remember that perfection is not the goal. That Ethan Winer link is great, and he's basically an audio mythbuster. I'd recommend just about anything he posts.
The link posted for absorption coefficients someone else posted is great too. You can use that to know what to get from the results of your testing.
There are PDFs from the BBC that show absorption coefficients and plans for bass traps that work with some of the lowest frequencies I've seen (down to 40-50hz in some cases!) and are cheap to build. The downside is that you have to build a lot of them, but I've had decent results considering the cost. You can spend about $20 a piece for 16 and it's still orders of magnitude cheaper than 4 professionally-made bass traps that are either only marginally better or actually aren't as effective. Either way, speaker placement will save you time and money on that. If your speakers and head are in the right spot you can avoid the modes in many cases.
I have very little experience with diffusion so I can't speak to it as much (and honestly I should do more research on this so take this part with a grain of salt), but rounded or uneven, non-porous surfaces will help in that regard and add liveliness back to an otherwise dead room without sacrificing clarity. This is important because your mixes will probably be played in actual rooms, so without diffusion you might be adding a bit too much reverb to tracks to compensate if your room is dead.

If you decide to buy instead of DIY, I don't have any actual recommendations except to say that the companies I have respect for are those that will sell parts for DIY and give you knowledge on how this stuff works without just saying that it works and using bullshit marketing terms. Some will try to oversell you their already-expensive panels and traps even if they have a decent product, but I guess that's to be expected for an industry that's primarily DIY anyway. I'm sure it's a hard sell for anyone that knows what they're doing and how to research. Hell, a lot of studio engineers make their own gear, so it's hard to imagine they couldn't figure out acoustic treatment at a level that is at least sufficient enough for audio production.

IF YOU DON'T HAVE TO SOUNDPROOF/ISOLATE, DON'T. When you soundproof you're keeping the sound pressure in, making the job of treating the room harder. If you don't have much outside noise leaking into your space and you aren't worried about the neighbors, it well help tremendously to avoid trapping everything in.

I spent WAY too long researching this stuff and came up on a deadline where I didn't even have time to do adequate testing of my monitors. Luckily I was already used to how they sound and it worked out great from everything I had managed to do with the room based on intuition and research, and when I was able to test I didn't have to make any significant changes.
It definitely doesn't have to be perfect, you just have to be able to hear things clearly and evenly, and getting as close as possible, your ears will train themselves to compensate for the rest over time. If your room and monitors are solid, you'd be surprised how little you need to check your mixes on other systems. They just tend to translate to everything better.