Out of all the monsters in the MM1 with completed statblocks (so excluding dragons, since those all have to be customized by the DM anyway), the Solar has the highest Spot modifier of +32.
Kobold Rogue 20.
Dex of 30 (16 base, +2 racial, +6 enhancement, +5 inherent, +3 levels).
Dex mod of +10
23 ranks =+33
Hmmm, before counting size modifiers or any direct Hide boosters, we're already beating the highest Spot in the MM.
+8 Size, +2 Shadowsilk leather armor
Up to +43 modifier now.
+2 masterwork tool, +15 competence (we'll go with Shadow armor property, since it's core)
Greater Collar of Umbral Metamorphosis +8, HiPS.
Hmm, +58 modifier right there. Decent chance of beating the Solar while attacking and I've only grabbed three things outside of core. Skill Mastery means we always get 48 on the hide check while attacking as-is (the Solar has to roll a 17 or higher to see the Rogue and a 37 or higher if he's not taking penalties for his actions), but why should I really be stopping here? Would you like me to continue? Keep in mind that only the inherent bonus is really any significant amount of money and I have a ton of things I can still dive into. Note that this is still ignoring spellcasting and even the Dex isn't maximized. There's a LOT of room to improve with minimal effort.