I would not recommend to wait too long with the multiplayer part. It is easy to turn on a few more clients (and just ignore them if you don’t need them). There is already a lot of replication working out of the box. For example if you start a blank third person template with a second or third client then you would see your characters running and jumping on all clients without any requirement of replication from your side (so far). But if you make progress and add some more elements like spawn this or aim or move there, … then you could see what would work already on all clients and what requires a bit more from your side. It is not just replication but it even differs if you would prevent cheating where you would do some stuff.