Ads(like on the blank space on the page or owner paying to show their house on the top) and donations is what I thought for the server costs, but you are right marketing is an issue
Want to solve the problem of paying to get the owner's number which am facing myself, It's too much of work to be just an hobby project
Initial plan is to not verify anything, the issue getting solved here is the tenant can see the photos of the house, if they like it they can get the owner's number without paying
After this it's between the owner and the tenant, the tenant should go and do a propery visit before taking the flat