@lemon I thought of one other device that might fit your requirements, the Hoxton OWL pedal:

http://www.rebeltech.org/products/owl-pedal/

The hardware is open source.

You might also like this page on the Axoloti site where the designer compares his board to other products - useful starting point for research:

http://www.axoloti.com/more-info/comparing-axoloti/

Apologies if this is totally outside what you're looking for, but have you seen the Axoloti Core board?

http://www.axoloti.com/product/axoloti-core/

It doesn't run Pd but it has 1/4" jacks and uses visual patching software that might have an easier learning curve, if you already know Pd.

There's also the Teensy family of Arduino-like boards, which have an audio board available and another visual patcher:

https://www.pjrc.com/store/teensy3_audio.html

I'm also hoping to build a Pd-based desktop synth. From my research so far, a Raspberry Pi 3 + some kind of Arduino (or a Teensy) seem to be the best bets in terms of support and documentation. Although as David says, they won't be as cheap as a secondhand laptop.

After some trial and error I was able to get dssi 0.96 to compile and run on my Linux machine (Lubuntu 15.10). Here are the changes I made to the files:

• In the Makefile: changed "LIBDIR=/usr/local/lib" to "LIBDIR=/usr/lib"
• In the Makefile: changed "current: pd_darwin" to "current: pd_linux"
• in dssi~.c: added "#include pthread.h" on line 31

The first change might be optional, depending on how your filesystem is set up.

The help patch and output~ abstraction in the doc folder also needed a few changes. (The first thing I did was to cut the connection from the sine plugin to throw~, in Instance #2.) I got Hexter to load by changing the dssi~ object in Instance #1 to "dssi~ hexter"; it didn't seem to need a path. And it needed to be connected to throw~.

After that it seemed to work fine. Selecting channel 1 (it's at the bottom right of Instance #1) got sounds out of Hexter.

A few things to look out for -- clicking on "reset 1" or "dssi show 2", or trying to create a plug~ object, crashed Pd. And I had some occasional problems with Pd hanging until I killed the hexter_gtk process, usually with that big help file. A fresh patch with a dssi~ object doesn't seem to crash as often.

I poked through the dssi~ C file and saw something like a "deactivate plugin" signal you could send to it, but can't figure out how to make it work.

Anyway - hope this works for other folks. Have fun!