Those Doctor Who stories are strictly for completists, jhal. Pretty much like the Who tales penned by Alan Moore, they're not necessarily bad, but don't go around expecting to find some forgotten masterpiece. (I'm assuming you're going to read the recent colorized reprints from IDW? )
If you want to read some early Morrison before tackling The Invisibles, I would recommend Zenith, which is actually as good as anything Morrison ever wrote, but if you don't have the original 2000 AD Progs you might have a problem locating the complet story.
Sebastian O was cool. So were the two Seaguy minis, but as you know, those came after The Invisibles. (Oh, and I actually liked Kid Eternity a lot more than 5YL)
For that matter you might as well reread The Filth, which actually works as a thematic coda to The Invisibles.
Flex Mentallo is beyond awesome, but I would read it after reading DP (for reasons that will be obvious once you get going)
Any reason you didn't include his Animal Man run? Or Arkham Asylum? Those came slightly before and during his DP run. (Perhaps you recently reread those? )
Other fantastic early stuff (but again difficult to find these days) is The New Adventures of Hitler, St. Swithin's Day, Dare and Bible John.