Re:
As Clubby Says, it's just a 700c/29er rim, and all but the narrowest rim will be happy with a 40c tyre.
I'm using some 27.5/650b mtb wheels on my bike, a bit weighty but strong enough to tackle some silly singletrack stuff.
If you want a bit lighter then look at cx rims, Stans Grail etc...
You Could use road rims for even less weight, but they're generally narrow and lower spoke count ie not as strong, but I've used road wheels on fire road tracks etc and not had any issues.
More important imo is that the rims are Tubeless compatible, so you can run lower pressures in the bigger 40c tyres, and it saves on inner tubes and patches!