Peripherals in a setup can make or break the aesthetic. However there is an infinite amount of possibilities and a lot of the customizing comes down to personal preference and what works best with what you have. What helped me make my decision on most of my current peripherals was looking up reviews on YouTube. I will provide some general information about the factors to consider when purchasing peripherals.
Whether you want to build a keyboard from scratch or just purchase one at the bare minimum make sure it is mechanical. The next consideration to take in is what switches you want, this is personal preference. There are three main switches brown, red, and blue. With red switches there are a middle ground, not very loud and smooth. Blue switches are extremely loud and very "clicky" this is what I prefer and it feels like I am writing on a typewriter. Brown switches are tactile like blue switches meaning they have a bump to let you know that you have pressed the key. The last consideration is whether tactile or linear is best for you, linear will be smooth with no bump.Other considerations include wired or wireless and what size you want the keyboard to come in.
Mouses are pretty straight forward. The preference whether you want wireless or wired and what fits best for your hand. Additionals considerations include how many extra buttons you may want and the main consideration is what sensor. The two sensors to choose from are optical and laser. Optical will use an LED to reflect of the surface below to track movement while laser will use a laser.
The main thing to look for in a headset is 7.1 surround sound. As long as it has good sound and is comfortable to you not much else matters. Some may prefer a control panel but this is not necessary. Just make sure whatever you choose is compatible with your system, PCs should have a USB and audio jack, while consols typically wont use a USB connection just the audio jack into the controller.
Many third-party companys create customer controllers that can take your setup to the next level. SCUF is on of the biggest third-parties in this field. Just make sure the controller you purchase is for the correct system. My personal picks are the SCUF Instinct Pro, and Razer Wolverine Ultimate. I do not recommend the Xbox Eltie Series 2 as the build quality is terrible for the price point. Keep in mind my suggestions are for pro controllers and personal preference they may not be necessary for your setup.
The monitor decision will be affected by what your playing on. The resolution, frame rate, repsonse time, and panel are the 4 factors when deciding what monitor to buy. The resolution and frame rate directly correlate to what your system can output whether that is a consol or pc. The higher resolution and frame rate the better. Keep in mind the higher you go in reolution it is to be expected the less frames your system will be able to output ot the monitor. Repsonse time you are looking for 1ms and a panel that fits what your intentions are. A VA panel will offer better picture and IPS will offer better performance but with recent advnacements the differences between the two have become very slim.