I use an SD card to boot my eee PC 900A. It's a tad slower than the 4 gig
SSD, but it works fine. You can use the installer, just select "other". You
should see something along the lines of "sda 2048MB sdb 16384MB" and you
select sdb, click on the defaults, and go get yourself a cup of coffee while
it installs.
< warm smiles >
You will want an external drive to store your files, but there should be
enough room for the configuration files in your home directory.
Mine still has Trisquel 6 on it, but that may change by the time you read
this if the video card is up for Belenos, which my "nice" laptop's isn't.
You're inspiring me to revive it and go get some groceries with my "new
laptop" fund. :D