The T15 is by far the easiest to build and does indeed sound quite nice. And a fantastic price!
Next easiest to build in my opinion is the A12... but the C84 isn't terribly difficult either and would make a great choice for a pair.
Personally, I'm partial to the N72's, but that's partly too because I have other pre amps already to fill some of the other 'colors'. The N72's are a bit more difficult to build too I think, so maybe not a great starter. But, none are overly difficult if you have a bit of experience and are patient and take your time and follow directions.. including all the stickies here on whatever module you're building. You will enjoy whatever kit you decide to build I'm sure.
Best of luck!
Commodor