Hello Austen,
The KR5 was our first "own" robot, I remember it fondly, e.g. from Berlin:
https://vimeo.com/30506602That being said, the KR5 is - if I remember correctly - a Denso robot with a KUKA controller, so it's a bit of a strange combination. But it worked fine for us. That being said, especially as a beginner I would rather go for a used KUKA Agilus as it has got the newer KRC4 controller. Not sure how availability is in the US, though - you could start by calling KUKA US so that they can get you an offer. I would assume that a used Agilus with a few years on it should be around 10-15k USD.
For the KR5 you mentioned I personally wouldn't spend more than 3-7k on it.
Regarding PRC you could already implement it via the Custom Robot component, I can give you some pointers if you need help. I've attached a Rhino model of the 850 variant.
The controller should work fine with PRC. Regarding power, we always ran it from a standard 230V power outlet and never had any power issues - not sure if there is a US variant or if you need a voltage converter, though.
Best,
Johannes