Not every Glory of the Ulduar Raider is possible to solo. Some of them could possibly be doable in absolute BiS gear, but I will go over them:
Orbit-uary is apparently possible if your vehicles somehow don't explode, I don't know how though, I haven't bothered with it more than a handful attempts. Since my vehicles kept exploding, I just gave up on that.
I'm not sure about Stokin' the Furnace, could be possible with insane gear but I'm not entirely sure. I don't remember my usual time to kill Ignis but according to DBM, which on this harddrive only has 3 kills, my best is 4:47, but I could probably improve that. With absolute BiS gear I'm pretty certain it could be done.
I haven't tried Crazy Cat Lady, but I'm almost certain it's impossible. Her cats will absolutely shred you and your pet needs a cooldown up to tank Auriaya and her cats. The cats are too fast to do any sort of kiting, and they will jump you from ridiculous lengths. If someone is actually crazy enough to try this I would be interested in seeing the results
I Could Say That This Cache Was Rare could again be doable with MAX gear, survival spec and PERFECT play. It would need crazy execution since you basically have no self healing, you need to knock the NPCs around perfectly, stand in a light beam + close to a fire and keep up Singed. It would be insanely hard but probably not impossible with the best gear and perfect execution.
So yeah, 4 hard/impossible ones, rest are basically easy at least if you have decent gear. Algalon isn't needed for Glory of the Ulduar Raider, but if you are below 400 ilvl forget it, if you are at 400, be prepared to practice for 2 days
As for when things become doable / easy, I just WISH I had written down my item levels for each of my kills!

The only one I really remember is Algalon with 400.1 ilvl, but that was with a pvp ring since it was 25 ilvls higher than my pve ring, and the higher agi made it better.
In Mists it would be cool to do what you suggested though, basically write down ilvl and spec for each soloed encounter when done for the first time. One could even make a chart where it's kind of like a timeline, except instead of time it would be an ilvl-meter and then you had arrows showing at which ilvl each boss was killed. That way you could easily see which bosses would be doable with your gear and how hard each boss is relative to the others.