I also tour on an mtb. Specialized Hard Rock, 14 years old. Works great. No need for anything else.

On the handlebars, I would consider trekking bars. Flat bars will work, flat bars with bar ends are better, and trekking bars are better yet. I've toured with all the above, but always got numb hands until I got the trekking bars. Existing brake levers and shifters will work with them. They cost me about $10 USD more than the bar ends.

Saddles - tried lots of different ones. Finally got a Brooks B-17. After break-in it's great. Pricey though.

Tools/Skills - I would suggest learning basic bike maintenance skills, and carrying a multi-tool with you. Derailleur adjustment, brake adjustment, chain repair, etc.

Tires - almost anything will work, especially for a short tour. I toured on off-road knobbies before going to Schwalbe Marathons. The Marathons roll easier, but are pricey.

Good luck.