i've built several wheels using mix and match parts; i learned how using
http://www.wheelpro.co.uk/wheelbuilding/book.php
if your 105 hubs are in good shape and rims are not way out of true, then you can definitely make a respectable set of wheels; i've built some kid bike wheels using really crappy rims and gotten worthwhile results - they did need several re-truing efforts before the brakes no longer rubbed
save money? maybe; i built my own truing stand; made a nipple driver; bought cheap spoke wrenches and then got the spokeys later (get red if you get one and ride road bikes)
save time? no way, but that's not what it's about
don't expect anyone to be impressed that you built a set of wheels; normal people make an exceptional effort to avoid being drawn in to a discussion about wheel theory, measurement, lacing, dish, tension, and lateral and radial tension