Merlin Cycles currently have the Stans 355 and Olympic models at half price as they’re old stock. These are great rims. If you wanted the newer versions they are the Crest and the Alpine.
Your wheels will probably need new spokes, so factor this cost in too.
As for how easy is wheel building. Well it’s not rocket science, but it is one of those things that the more you do it, the better you become. I’ve built hundreds, and am a confident wheel builder. First few I built I always got someone else experienced to check them over before I rode on them. If you’re not happy doing it, likelihood is your LBS has a decent wheel builder working there (if you’re local to Worcester come and see me I’ll do it for you) who will do them for a reasonable cost.
Other thing to consider though is are the hubs worth keeping? Might you be better off buying a new off the shelf wheelset and selling your current wheels to help pay for it. My experience of Specialized is that they usually fit cheap hubs to otherwise high spec bikes as a cost cutting measure, so may be worth buying a complete wheelset and selling the current one on or using them as spares.