@popey: Sounds familiar. Did that with #Debian some years ago as performance at the #VCFe because I wanted to install the most recent version of Debian possible on a vintage #Thinkpad 760XD: Pentium MMX w/166MHz + 48 MB RAM
The big issue was that the Thinkpad did support a CD-ROM drive, but couldn't boot from it. So I had to start with the last Debian release which still supported installing from floppies: Debian 3.0 Woody
I then upgraded until Debian 8 Jessie—the last release running on a P1.