1) DME, if kept dry will store almost indefinitely. If you add more, you'll bump up the ABV. It's up to you.
2) Crystal 20 will give more flavor/body and color. As for style, that's up to you -it's YOUR BEER after all.
3) Nottingham should be good. If you are going to buy more grain/LME/hops anyway, a pack of packet of dry US-05 yeast (my go-to American Yeast) would add about $3 to your total.
4) Mash all grains at the same time. This is what all-grain brewers do every day.
One of the hard parts of homebrewing is making these choices. No matter what, it will be some of the best beer you've ever had.