Buildbot & RTAI ?

None of them are available via buildbot, They are Linux kernels, not LinuxCNC kernels.
But LinuxCNC is dependent on them so Andy ( i think ) was kind enough to provide us with the RTAI 64 bit version since to my knowledge it is no longer developed and maintained by the Linux community, only the Preempt-RT version is being developed and updated.

It is impossible to have a list of working PC's due to the wast amount of differences and different hardware used in them. Even the same Pc or laptop from the same company can have a wide variety of hardware choices.
As a general rule, the enterprise edition PC's from Dell, HP and Fujitsu do work nicely as far back as core2duo and up to 4'th generation of i3, i5, i7, and they can be found from 25 to 80 euro a piece used.
Stay away from "home" or "Business" edition, those will most probably not work.
As for new PC's it is a cr@p shoot, i have Ryzen 5 3600 with an MSI MB that is useless, my bro has a Ryzen 9 3900 with an MSI MB that is very good !

Are you going to be using a parallel port for machine control ?

There is always the option of using LinuxCNC in simulation mode, so it does not need real time kernel and can not drive the machine, but everything else works, like running gcode and having it visualised on screen, playing with settings, changing and monitoring hal stuff, etc.
It also has some tool changers and 5 axis machines visually represented so you can view what the machine is doing, again on screen on a separate window.
As for machine control, the cheapest solution is using parallel ports, and there are add on PCI or PCI-E parallel port cards costing from 15 to 30$, There are Mesa boards that can work as two or more parallel ports on one card, but are much, much faster and can be programmed for a lot of stuff, like: stepgens, PWM, encoder counters, inputs or outputs on any pin that is not in use by the fore mentioned, etc. They are also very cheap compared to what they are capable of, starting at 69$, if i remember correctly, and they come in PCI, PCI-E, or ethernet variants.