Jason, you have reached paralysis by analysis. I fully agree with both Davids and Anuj above.
I would suggest you take a 2x4 scrap maybe 16" long and your number four plane to your local Sjoberg's store. If you are happy planing the edge of that, just buy the fool thing and get on with your life.
I think 4x4 that I can get at the home store is lousy for legs. They cut that stuff out of the worst trees they can find. For just a few bucks more, look at 4x6 for the legs. The ones you want are free of heart center, FOHC, but there are going to be some on every shipment of #2 Doug Fir around here. When you look at the ends you should see quarter circles of growth rings. When you look at the four sides you want all edge grain, no face grain. If you can find two of those at eight feet you have nominally four possible legs at 4 feet each. FWIW I was able to lay out my joinery so I didn't have to cut through a knot to make a mortise or a tenon; and while it was expensive for firewood the scraps got swallowed up by my wood stove just fine.
The idea that you don't have the skills is total BS. That is a lie and you need to leave it behind to get anywhere in this hobby. Focus instead on learning new skills. If you can layout and cut simple joinery every thing +/- one eighth of an inch your bench is going to last longer than the house you build it in. You should check each joint individually of course. One famous published author said each work bench joint should fit "like a hat in a hallway" which i think is a bit overboard. Another one said you should be able to close each individual joint in a workbench by driving each joint home with a hot dog bun, and not damage the bun. I think the latter is a bit more realistic. If you lay everything out as perfectly as possible, use only the 1/8" marks on your rule and every joint can go home with a hot dog bun you are going to have tolerance stack, you are going to need a 8# sledge to close the last joint and it will be bomb proof.
I would suggest (using green Doug Fir, stamped WWP in a circle for Western Wood Products Association and S-GRN for wood kiln dried to 19.99% Moisture Content and D FIR inside a triangle) that you draw bore 1/8" and at first assembly with no glue and 1/2 inch double pegs at each MT joint. If you can't drive the pegs out later drill them out with a 3/4" bit, then rebore for one inch pegs with 1/8 drawbore after you move. When you reassemble it with the 1" pegs glue the snot out of it knowing it will never again come apart.
Tool well is a disputable matter, you are going to have to work out your own salvation on that one. If my wife was a woodworker she wouldn't need a toolbox because everything would be in the tool well. I have a place to put all of my tools away and have no use for a tool well, it looks like a saw dust holder to me. Just saying.