I think the basic answer here is to expect to "have to redevelop everything" when you add networking.
Adding networking will touch almost every part of your game - both code and design.
A lot of the difficulty comes down to dealing with latency (there's also things like packet-loss and out-of-order packets). If your game is real-time, you'll want to "hide" these network realities from the user.
Code-wise, this (in an extremely abstract sense) means making your simulation tolerant of inconsistencies - and then introducing a bunch of them. I can't really get any more specific than that without going into a very long and detailed discussion of how game networking works.
That that basically describes the same dilemma that you face: You can't guess at how to make your simulation network-ready without having a full understanding of how game networking works (and how it relates to your game specifically). And the way to fully understand it is to implement it.
Design-wise it's a similar story. There are several classes of gameplay ideas that will not work due to latency. Just one example is any competition where one player can be first, with immediate feedback and split-second timing. Player A might be 100ms slower than player B to hit a button - but because they are separated by 400ms of latency - they both think they won. So if your gameplay contains any scenarios like that, you would have to re-design them.
To summarise: It's perfectly valid to write a single-player game first and then add multiplayer later. One benefit of this strategy is that you get a working single-player game earlier. However, if you go down this route, then expect to have to treat your single-player game like a throw-away prototype when it comes time to make a networked version.
(Trying the opposite path - trying to account for networking without actually implementing it - is a gross violation of YAGNI.)
It's worth finishing by pointing out that making a networked game is easily an order of magnitude more difficult than making non-networked game.