+1 on the 4th hand tool, especially if you work without a helper available. As advised above just buy tools as you need them. Get a good cable cutter; Park or Shimano are good, Felco is expensive but the best there is. A cheap digital caliper is very handy. A chain tool and some master links are bike bag essentials. A proper pedal wrench is also a must.

I stay away from multitools even for on-road use; a judicious selection of hex L-wrenches and open end wrenches is probably lighter and certainly more functional. Trying to mount a bottle cage, for example, with a multitool is misery.

Grease is still grease, although I recommend Tef-Gel for joints subject to dissimilar metal corrosion, like bottom brackets and pedal threads. Pull your pedals, seat post and quill stem, if you have one, and grease them to avoid them corroding stuck. You will doubtless want to replace your brake pads; after all of this time they are probably dried and hard. Kool Stop Salmon are often recommended. Best not to ask about chain lube as is tends to create a LOT of controversy.