I'm trying to make an elf kensei III with 2 levels of rogue for evasion and UMD.

Since I just have base charisma, at level 20, my UMD with full ranks in it will be 22. Is this a useful number, or should I just forget about it?

The will save seems like the weakest part of this build. Is it worth is to take some feats and enhancements to try to shore up my will save? With Iron Will, Luck of Heroes, Bullheaded and maybe Discipline as feats and Elven: Enchantment Resistance for enhancements, I could boost my will save by up to 8. Is that helpful, or will I still only be saving on a 20?

Is it worth while to take multiple toughness feats? Each additional toughness will give me 22 HP at level 20, right?

Will Stunning Blow land enough to make it worth getting? Does Stunning Blow stick an enemy into an auto-crit zone? What about Improved Trip? That takes Combat Expertise and an int of 13 to get. Is it any good for an elf who only has fighter enhancements for tactics DCs and no racial enhancements?

Thanks for any advice!