Fenders area must, if you plan on riding in the rain regardless, as they help keep the grime out of the drivetrain and off the frame. Full wrap SKS fenders work better then the Zefel clip on type, but anything helps and type depends on the type of bike and whether the frame has eyelets to support SKS.
That and keeping the drivetrain clean and lubed (you can wait to do this at home, not after every commute leg), as well as keeping an old rag handy after every ride to simply wipe off some of the grime before it hardens on the frame and drivetrain.
Don't park the bike in the rain if possible, if locking up outside, try to park it under some cover.
If you do ride rain regardless, you will need to replace cables more often, no way around it, so it pays to buy the tools (cable and housing cutters, assorted metric allen keys, 3rd and 4th hand tools.) and maybe take a basic (or advanced) bike maintenance class at a LBS. Saves you money and time not having to pay a shop to do it and you can buy housing and cables in bulk and save money