It depends on what you are streaming. If you are gaming, you can get a really decent mid end PC (and peripherals) for less than 2000.
If you get 2 PCs (one for gaming and one for encoding) then you can also get really great quality that way.
Of course if you are not gaming, then you wouldn't need anything nearly as powerful.
As of being 'Optimized for OBS' there really isn't anything to do since OBS is designed to be very versatile and run on almost any equipment.
The most important thing for video encoding is the CPU by FAR. For just gaming PCs you don't need an insanely great CPU since your GPU will almost always be the bottleneck and doing most of the work.
For video encoding, that is generally on the CPU. If you are just doing a local recording, using your GPU is fine since you can record to a much higher bitrate and don't have to get great compression which will cause your PC to run MUCH quicker.
To get great streaming quality you don't need a high end computer especially because people watching live streams come to expect less than perfect quality. That said, if you don't care about value too much and just want the best quality you can get, going for some insanely overpriced xeon would work.
Also if you tell me what you are going to be streaming/recording, and what other equipment you need, then I could much further help you along.
Long story short, its all about the CPU, and very situational. One thing to note is, as OBS is designed to use multiple cores, it (like most other programs) runs much better on fewer faster cores than more slower ones. (*basically don't buy some insane 22 core xeon*). I wouldn't recommend going more than 8 or 10 cores.
