If you're OK with disc covers they should be alright, they won't be much less strong as a complete wheel but losing a spoke can be more of a problem as there are less other spokes to take up the slack, if they're very cheap then go for it, but if it's just a bit cheap I'd hold out for some 36.
If you're OK with disc covers they should be alright, they won't be much less strong as a complete wheel but losing a spoke can be more of a problem as there are less other spokes to take up the slack, if they're very cheap then go for it, but if it's just a bit cheap I'd hold out for some 36.