A straight neck, proper intonation all along the neck. You don't want fretbuzz either.
hmm one thing I would make sure is not to buy a plywood uke like the mahalo's.
Some people would say this wood has this tonal quality and that wood another, but I'd never really pay much attention to that. I'd just want wood that has dried well over some time before the uke was built out of it, and I want it to look nice.
If you take a cheap plywood cigarbox for instance and compare it to an old mahogany box you can clearly hear the difference.
Last but not least make sure it has good geared tuners. But you can also buy those seperately and put them on yourself.
Concidering alot of ukuleles are good enough to begin with, you might just want to go with your gut feeling. When you see and play a uke and your gut tells you that you have to have it, your gut feeling is usually right.
But you already know all this right?
I just noticed you're a senior member already.