I am with GarryRicketson as far Sid is concerned. I think you said you run openbox, so no full DE, which makes it even less likely to break.
Thing is: it can break. If someone wants to avoid that at all cost, then Sid is not the right choice.
I for one avoid Sid on the ppc (as of yet), as i am really not good with how i would do most basic troubleshooting on the powerpc (say reinstall the bootloader, yaboot?, how to chroot it, main problem here would be to find a working live CD, and re-installing and configuring can take quite some time. On a PC i do it in what? half an hour? like that).
But ! : as you only have the choice between old-stable/jessie, which very soon won't be even old-stable anymore (but what? stoneage-old-stable?
) or Sid i think it is worth a shot.
Or, as many have already said, switch to OpenBSD right away (i guess that is what i will do in the future).
If people will complain: oh, ah, uh, why the heck you running Sid, you can answer: i go no choice.
Sid: install apt-listbugs, install apt-changelog (?), run updates really often, once a day won't hurt, once a week i would consider the absolute minimum.
My two cents, good luck with your original problem (fonts), of which i have no clue.
You sure you are running the correct graphics driver? (i guess yes, but enter the graphic card model as a searchterm at wiki.debian.org anyway. Perhaps it will show a minor detail you are missing regarding that card when using it on a powerpc. There are quite some pages, but they are not easy to find).
for your lulz: for me jessie makes pretty much all web-browsers segfault. Firefox doesn't, but it is way too slow to be usable, netsurf and dillo both work (but ... well ... i got used to it). No sound before days of searching and fiddling, etc. It sure is a challenge to use a ppc ...
, but the good thing: you are not alone, and it sure isn't you who adds problems, which others won't easily understand
Anyone who makes the OS boot at all is a winner in my book