700' s are great! But try to get 36 spokes at least- there's a lot of 32 spokes out there, especially in flip/flop wheels
also recommend a 1 inch or so wide rim as oppossed to 3/4" because you can't go more than 32mm wide on the narrow rims- although my 50 has had those on them for several years- the back tires wear a bit faster-
having raced on the road, I like them biut I'd recommend about 35 to 38mm wide- lightweight and fast rolling - large profile for bumps!
Just replaced a 27" back wheel on my 66 build that kept breaking spokes with a700c flip/flop hub and alloy sprocket bolted directly to the side- really lines up great and LOTS of chain stay clearance on the frame- no tensioner!
so both my builds are direct bolted on sprockets to high flange smooth sided flip/flop hubs I drilled myself in the six bolt disk brake pattern.
Disk brake rear hubs are available to, but I think they'd put the sproket out a bit more towards the frame, and I'm not sure how well they'd lock on
On this last one I used titanium bolts and double nuts- the whole assembly saves all kinds of weight over a normal steel sprocket and rag joint.
with a lightweight 415 trike/industrial chain and no tensioner- it's a whole lotta weight i'm not lugging around- and the chains roll better too.