Adding replication to a single-player game sounds like a nightmare to me. The more complex the game, the closer to impossible you will get.
Multiplayer is definitely not something for new UE4 developers to attempt. Heck, there are many seasoned devs who still find it too much to deal with to be worthwhile. It would be a much better use of your time to learn UE4 without the complications of replication (IMO). But if you do, I recommend starting your project as a multiplayer game rather than porting it later.
Hope that helps.