well I dont have any experience with the ninja, so I'll just pretend you said monk
I think you could go with maxing out the mental stats and letting the physical stats slide. Get a few good feats based on burning turning attempts and get something to shift your form to other creatures forms whenever you need to do something. Mold your body to the task as needed as any good monk/ninja/whatever would.
I would even suggest, if you are starting at decent levels for this, to be at least one age category up as well. Older and wiser you have learned to shift yourself to fit the exact situation at hand and are infinitely versitile.
There are several items that will help with this. Although at very low levels it will likely be a bit tough to surive.. although your saves will be insane!
Edit 2: duh on me, I just noticed it said 5th level..lol.. so 9k gp to spend
Edit: I guess we could assume 32 point buy for now as the next poster did
So I would suggest this (if you are limited to core races only):
Str: 9 -3 (age) = 6
Dex: 8 -3 (age) = 5
Con: 9 -3 (age) = 6
Int: 16 +2 (age) = 18
Wis: 16 +2 (age) = 18
Cha: 16 +2 (age) = 18
Human feat: Improved toughness
4k for an amulet of health +2
Be skillful and sneaky. You will need a lot of sneaky skill to overcome the dex, good thing you have that high int
you arent quite high enough yet to have paladin turning, but you are nearly there.
Get a good mount and a few other items to help your surviveability and pick up any and all items to help you shift your form and you should be set.
I am not saying this is a great characer, but it is one I think would be fun to play.