Yes and a topic that I like very much.
We're in 2020! Like I always say, we sent a spacecraft to the moon with a 2.048 MHz computer
-Ubuntu Studio, maybe an RME PCI card
Why RME PCI, you can find good external soundcard, I guess it depends on the computer that you will choose (more on that later). Yes I like Ubuntu Studio, good choice.
-Really only doing audio (oscillators, arrays, filtering, delays) No graphics.
Perfect, start pd with -rt -nogui
Use [pd~] only if topping 100% CPU (pd is single thread).
-Keeping the the machine quiet (low fan noise) is VERY important.
Fanless is possible, again depending on the computer you choose.
What CPU specs matter most for common audio and MIDI tasks in PD? Number of cores? Thread count? Clock speed?
Clock speed = lowest latency (you can push jack to buffer 64) without xruns. If you are not playing live (for example using ADC) you don't need low latency configuration (I am lucky and not very good at detecting latency, my setup is around 38ms (round-trip). You can detect latency using jack_iodelay.
RAM is important if you want to load samples in PD in advance (avoiding glitches).
NVMe SSD if you can.
If I run other apps (VCV rack, Carla, various Jack plugins) will those processes distribute to the other Cores?
Yes, again Pd is single thread. Others are usually better (GUI on a separated thead for example).
Does Pd benefit from a more powerful GPU card? Or will there be no difference if I use the GPU embedded in the CPU? Is it different if I launch Pd without the gui? (-nogui)
If you don't use Gem you don't need a dedicated GPU card.
Here's some ideas for you, I've been building some setup over the years:
Theremin à crayon:
Using a Surface Pro 3 running Ubuntu Studio with a "old" USB 1.1 sound card. Heavy patch using lots of software : Bitwig, SooperLooper, Guitarix and of course PD. Midi (PD), OSC (Bitwig, SooperLooper). Very quiet but the Surface gets hot (fans are kind of quiet like a good laptop).
Similar setup, a quiet PC using a big heat sink and a overrated power supply so the fan never start. Noctura fan on the side (expensive but quiet). Old soundcard (firewire) but I can do low latency. 8 ins/8 outs.
Biscuit box computer:
Mini-pc not quiet, not very fast in this case a cheap usb soundcard (you know +- 8$ barely better than the embedded one).
A raspberry pi 2 with wolfson audio card. Using only PD with Alsa, I am getting very good result (low latency) quiest setup. But of course I need to be careful with the CPU.
Working on a new project, I found this midi keyboard in the trash / snow. I will put Khadas VIM version 1 (ARM) with a BEHRINGER UCG102 (usb soundcard for guitar). Quiet, no fan can be run on a battery (5V). Will post the project when over.
Never worked with it, but looks very powerful. There's a price tag. Maybe for the next project.