Thanks all - plenty to think about
The gearing should be ok as the group I'm looking at has a 48:11 as top gear. He'll never use the granny ring but it's a cheap solution! Swapping the chainset out for another would push the build out of budget.
Having just researched further, both road and MTB frames usually have a 68mm BB shell, so assuming the front mech fits, I reckon it should be ok. The group set has an old skool internal sealed bearing type BB.
I thought about using a MTB frame but it needs calliper brake mounts and clearance for 700c wheels. It would also be heavier and burlier than required.
The geometry's the tricky one alright. I've converted drop bar bikes before and they've been ok, but each frame is different. They're not always designed differently, as the Specialized Roubaix drop bar bike has identical geometry to their flat bar Sirrus, but the frame I've had my eye on might not work quite as well...
I think I'd better use the geometry of the bike he had nicked as a guide and then tweak the position with stem length.
Thanks for the help folks!