In the next autumn/winter I would like to try to set up a self hosting
solution

And I'd love it to be GuixSD based

I need some help with selecting the hardware that fits the goal
Hardware is not exactly my thing

A Beaglebone board (or equivalent) ? An Intel NUC (or equivalent) ?

It should probably serve a collection of static files over https, have a
smtp server and an IRC bouncher.

Optionally I'd like it to run some bittorrent or rsync or synchthing or
ipfs or DAT and a Gnunet node.

I'd love to be able to attach some external hard drives to it, both
traditionl hard drives and SSDs.

In the last few years the ADSL that serves my apartment has improved
dramatically

It used to be 300 Kb/s in upload for years

Now it's around 19 Mb/s

The optic fiber cord reached the cabinet just aside the building main
entrance

The copper fragment is now only a few meters.

So I could use some hardware suggestions to set up such a solution based on
GuixSD

It has to be cheap AND moderate in electricity consumption, possibly like a
home router (or something)

Thanks for any hint