Assuming the arcane spellcasting class has a poor Fortitude save and Bluff as a cross-class skill, you need a single level of anything that has a good Fortitude save and Bluff in-class. Then the +1 of your arcane class’s poor Fortitude save after 3 levels will add with the +2 of the good Fortitude save from the new class, and you can just dump all your skill points in Bluff (ideally, a high-skill class makes this easier, though if your arcane class is intelligence-based it may not matter).
You still need 5 overall levels to get the Knowledge (religion) and Spellcraft ranks, however, so that is a hard limit. You thus want 4 levels in whatever your arcane spellcasting class is, plus one level of something else.
Many classes fit this description, but none are likely to work as well for you as savage bard:
Good Fortitude save
Bluff, Knowledge (religion), and Spellcraft in-class; no need for any cross-class ranks
6 + Int skill points, making it trivial to get the 6 you need in Bluff
Good 1st-level class features overall
- Bardic Music means you can qualify for sublime chord, an arcane prestige class similar to ur-priest in its quick progression. You’d still (just barely) get 9ths from your typical prepared arcane spellcasting class, so this is kind of unnecessary, but it’s a neat alternative, and it does hit 9th-level spells a character level sooner.
The order of your savage bard levels and levels in the other class is going to have to be a calculation on your part. If you take savage bard first, you get to multiply its base 6 skill points by ×4, which is probably better than your other spellcasting class gets. However, you’re also limited to 4 ranks in any one skill, which means you are going to have to put cross-class ranks in Bluff to get the remaining 2 ranks – costing you two more skill points than it would otherwise. Mathematically, savage bard first is better (6 additional skill points if your other class has 4+Int skills, 14 additional skill points if your other class has 2+Int skills), but you’ll have to make sure you can get all the ranks you need with your lower-skill-point-per-level class having to pay cross-class prices. Especially if that class does not get Knowledge (religion) in-class.
Because bard is Charisma-based (as is sublime chord, if you go that route), and you only barely get 9th-level spells from a prepared arcane spellcasting class (and wouldn’t from a spontaneous one), sha’ir from Dragon Compendium might be a better choice than wizard or sorcerer. It has you prepare spells (sort of), and thus gets new spell levels “on time” unlike the sorcerer, but it’s also Charisma-based. It does get a small amount divine spellcasting, though, weirdly enough: ur-priest requires that you forsake this ability entirely, but it’s no great loss.