For a bike with Hydraulic brakes you will need one made in the last few years.
A flat bar Gravel bike would fit the bill quite nicely . You can always add Shimano Hydraulic brakes , the low end ones for about $ 60-70 each.
Also You can buy a drop bar gravel bike then the flat bar shifters , flat bar and MTB brake levers/brake set .
Since the Specialized Roubaix has gone though a few changes in the last three years many people are trying to get rid of their older ones.
San Francisco Cragslist will have something , you have to check often though I have missed some good bikes by letting a few days go by before checking back on it.