Welcome to Centurion World, and Bike Forums in general. Great deal, by the way, on that 1986 frame.

"...does anyone have any advice for doing this?" I'll toss in my .02 worth, feel free to ask for a refund.

"...how much difference is there between the 105 and 600 components?" You already have the main difference, the 600EX crankset. It's a much nicer looking piece. The 600EX stuff is polished, the 105 is generally not nearly so, but it can be polished. The 105 calipers, I like better. Price-wise, 105 is cheaper. No matter what you get, I'd keep the crank arms if they're not dinged up.

You're fortunate that you don't have wheels, because you can go 6 or 7 speeds and get the wheels to match the group. Wheel sets of that era are out there, and very reasonable, and very good quality for the price.

I doubt you'll be able to find the DiaCompe levers with the red hoods, so feel free to get what's out there. I just happen to have some minor experience with that make and model, and may have some parts lying around.

Don't jump on parts if you have any doubts. Ask here, and one of us can help. geekrunner is in Indiana, and the market there seems to be very nice. Trying to go all OEM will be expensive, so I'd go with what works.

If I can't answer your questions, I'll either lie convincingly or tell you who to ask, instead.