Home charging is the most important thing to plan. I got by on 120V for a while but it was a little nerve wracking. The car will come with a Universal Mobile Connector, so you do not need to buy one. Just install an outlet that is appropriate for your needs. The various plug adapters are listed here, and the description for each one says how fast it charges.
Tesla — Model S > Charging and Adapters
Expect the Model 3 to be more efficient than Model S or X, so it will get a few more miles per hour than the description but not a huge amount.
Unexpected cost for me was an upgrade to my home's electrical service because the panel was full, by both number of breakers and load calculation. The upgrade involved replacing the conduit going through the roof so it was not trivial.
Also unexpected were Xpel paint protection film, Opticoat coating, and Crystalline window tint. But it was a very expensive car so I wanted to protect it. That sort of protection is much less important for a Model 3.
There were no unexpected closing or registration costs for me. Insurance cost was comparable to the car that it replaced. If your carrier tries to charge significantly more, then shop around.
As part of the preparation, take a look at plugshare.com for public chargers in your area. Check the descriptions to find out what network they are on, and consider joining those networks in advance. Some of them require a physical card that has to be mailed to you. The car will come with a J1772 adapter, so that is the standard you are looking for. Ignore the orange markers on the map, which are CHAdeMO and CCS high speed chargers, until we learn what the Model 3 comes with.
Final note - upgraded chargers on the car have no effect on supercharging (or CHAdeMO/CCS) charging speed. The chargers are AC, and are bypassed when you plug into high speed DC charging.