I've written a tutorial on the communication between Pd and the Arduino. You can find it here http://drymonitis.me/code/ it's called "Arduino for Pd'ers". The very first project is about blinking an LED controlled from Pd.

Pd runs on all three OSes, so a mac is only a prerequisite if you want to work with a mac. I switched from mac to linux about a year ago and don't feel the need to go back. On the contrary, I'm fully satisfied with my new OS.
If you don't want to spend too much money on a new laptop, I would suggest to buy a cheaper one that is very likely to have similar specs with a Macbook Pro, only it probably won't be in a metal housing. Focusrite Scarlett sound card series seem to work flawlessly without any effort on Linux. I've used a Scarlett 2i4 with an Odroid-U3 and a Scarlett 6i6 2nd gen with my Debian Jessie XFCE on my Lenovo laptop, both without breaking a sweat (with Jack).

These are my two cents.

@pepedlr not being able to save if Pd is not launched by root sounds like a bug to me. Did you install via the synaptic package manager? Which version of Pd are you using and which Ubuntu?

Since you're running jack as user pi (it's on a raspi right?) but Pd as root, there's a chance they can't communicate. I'm not sure if what I'm saying is logical, but it sounds like a potential problem to me.
Why run Pd as root anyway?

Do you mind sharing how? Maybe someone in the future will need this information...

Maybe a Windows user can answer this, I don't think I can help you with that, sorry.

As @whale-av said, the sound card you're thinking of buying doesn't provide 6 outs. I have a Focusrite Scarlett 6i6 and it does give six outputs (though 5th and 6th are digital outputs and I haven't used them yet). Its price is quite higher than the Behringer one, but the quality is much better too.

(1) Many sound cards come with a configuration software that you can use to set it up. You'll might need this to set all six outputs (with the Focusrite one it's not necessary, Pd will probably work out of the box).
(2) No special cable is required, that's what USB is for, to transfer data between devices.
(3) Audio interfaces usually appear with their names, so it's pretty hard to confuse them, and once set in a software, there's no reason why the software should confuse them either.

If you're going to use an embedded computer like the Odroid or the Pi, I guess you'll build some kind of controller, or are you going to use a computer screen as well?
In case you don't use a screen (which is meaningful when using and embedded computer), get rid of all the GUIs (sliders, number atoms, knobs, bangs, toggles, radios, VUs). Use [float] instead of sliders, number atoms, radios, and toggles, and [bang] or [t b] instead of bangs.
Get rid of all your array graphs and use the [array] object or [table] instead. Get rid of all the GOP (Graph On Parent) and enclose your processes in subpatches or abstractions.
This should save quite a LOT of CPU.

[dac~ 1 2 3 4 5 6] is indeed correct, but you'll also need to set the number of channels in your Audio Settings...

You're missing quite some objects. You're actually missing these three libraries: zexy, mapping, and cyclone, and you're also missing the comort external, on which the [arduino] abstraction is based.
Go to Help -> Find externals and type the names of the libraries (one by one) and also "comport" to install them to your system.
Then go to Preferences -> Path, and set the paths to the new libraries so that Pd knows where to look for them. Lastly, go to Preferences -> Startup and write "zexy" (without the quotes) so that the zexy library loads on launch (it's a very helpful library that you'll probably need for other stuff too).
This should get you going.

Note: You're using an old version of Pduino that's not maintained any more. I suggest to switch over to the one Roman Heafeli started supporting, which depends only on [comport] and no other externals. You can find it here https://github.com/reduzent/pduino

Since you're resetting [phasor~]'s phase, which happens in the control domain, rather than the signal domain, you'll most likely to get clicks. That's why I suggested lowpass filtering [phasor~]'s output with [lop~ 20] (or even [lop~ 50], the higher the frequency the more responsive the output, but the less smoothed).
This will smooth out the sudden changes. So, then connect [lop~ 20] to [s~ phase], [s~ phase2] and [unsig~ 20] (well, [unsig~ 20] doesn't really need to get a smooth phase change).

Without having seen your patch, I can imagine a sudden phase change can create clicks (or pops). Perhaps you can smooth out these phase changes by filtering the phase with a [lop~ 20] or maybe a lower frequency.

@LiamG I think you mean Ctl-click, not shift, right? For macOS it should be Cmd-click.

Yup, that's vanilla.

You need to find the absolute difference between your two original values, as well as the absolute difference between your two target values. Divide these two results and multiply your input by that result, after you offset it to zero (in your case subtracting 15), and then offset it again according to your desired range (in your case 0).
Check out the [map] abstraction from here https://github.com/alexdrymonitis/miscellaneous_abstractions
That does it.

E.10.complex.FM from Pd's help browser includes an FFT analysis of a complex FM. I guess you could use that do detect the loudest frequencies of your FM patch (along with the [array] oject).

The Serial.write() function writes raw bytes to the serial line. You can build your own protocol to receive and parse these values in Pd.
I've made two vanilla abstractions, one that works with Arduino's Serial.print() and one that works with Serial.write(). You can get them here https://github.com/alexdrymonitis/Arduino_Pd
There's one Arduino sketch for each abstraction explaining how to write your code in order for it to work with the corresponding abstraction.

Thanks, just did!

Hi,
I would like to test it.

Cheers,
Alexandros