A marking knife is used for marking where you will be cutting with your saws and chisels. For getting into tight spots (like dovetails) and making very accurate lines (which is vital for tight fitting joints) you need just the right marking knife. You would think that any old knife would work, but you would be wrong. Years ago I purchased several that didn’t work well.
I’m not sold on the need for a power jointer for flattening a surface. That said, I do have a Shopsmith 4″ jointer.. It’s great for jointing edges, and perhaps flattening the occasional rails and stiles, but it of course is inadequate for surfacing wide boards. Would a six inch jointer be better…..not by much. So what do we do? Go to an eight inch, or better yet a ten inch jointer? Now we’re getting into really big, heavy, and electrically hungry machines that are not really suitable for the small shop that is likely to be in a small shed or garage.
Wooden mallets are mostly used for hitting your chisels when cutting joints (like dovetail joints or chopping mortises). You should never hit a chisel with a metal hammer. Build or buy a mallet that is made of fairly hard wood (e.g. maple, oak, beech wood, etc.), that will feel well balanced in your hand, and that gives you enough weight to chop with your chisels.