Synthetic speech is nowhere near ready to be used as a replacement for real speech. The computer has no way of adding inflection to sentences. A couple of programs will recognise when there is a question mark but I reckon that you'd be much better off getting some voice actors.
The trouble with that though, is that everyone will record their bits in different environments and you'll also get inexperienced people sending you files with shocking amounts of echo and hiss, not to mention the horrible and in most cases impossible to get rid of, static created when they blow all over the mic when they speak. My advice is to get good at accents, find out how to change the sound of a file in subtle ways and to do it all yourself.
Good luck in sorting it out though.