You probably wouldn't need to redevelop anything. FileMaker Server is designed to open and host the unmodified files from FileMaker. It can even host unmodified runtime files if configured correctly.
There are just a few issues you have to look out for.
1) Global fields, by design, behave differently in a hosted file (Ether hosted by FileMaker Pro or by FileMaker server).
2) Any references to files external to the solution may need to be updated, not because you upgraded to server, but because relocating the file may require a different file path to maintain the connection to the external file.
So it would be possible for me to develop a runtime solution in either FM Pro or Advanced initially for standalone use but later get it hosted?
Yes. The structure of a run time solution is the same as any other FileMaker file. It just has some data added to it that "binds" it to a specific copy of the run time application. The server has a special setting for these files as they will have file extensions other than the expected .fp7.
Just keep in mind that each client has to have either FileMaker Pro or, if file is published to the web, a web browser, in order to access the file.
Also keep in mind that your stand alone runtime won't network at all. (can't function as a client of server hosted DB's).
Is there any way with FM server (and using a different runtime version that that developed from FM Pro Advanced) that users can download a runtime version of an FM front-end that communicates with the calculation engine/ database on the server? ie. Is there any way to get around requiring users to have FM Pro while having the solution on a server?
It is possible, however to publish from server to the web and then people with web browser can access the database from their browsers. Wonder if you could use the "web kit" from Internet Explorer or Safari to create your own web browser that would serve as the front end?
V. helpful insight, thanks.
If we used the web browser as the front end, can FM Server ensure only paying users get access?
I don't know. Password protection can of course be used, but as you have indicated in other posts that doesn't keep them from sharing passwords with others.