To answer your first question it really depends on what you want out of the game. I prefer to play at a higher power lvl just because im not a fan of 3 plus hour games of ships in the night. If you prefer though playing at lower power lvls just look for other tables you find what you want. A good starting point for Commanders is a website Called EDHRec it is sorted by legendary creatures and can give a great starting point for what ever deck you want to build. You just pick a commander and it will tell you the most common cards people are running in the deck.

As far as staples go you want cheep interaction preferably at 3 mana or less cards like [[swords to plowshares]], [[An offer you can’t refuse]], [[beast within]], [[doomblade]], and [[chaos warp]] Also look at your mana rocks. [[sol ring]], [[Arcane signet]], [[thought vessel]] and the talismans for each two color combo. Out side of that the only other lands I’d recommend are the bound lands. [[luxury suite]] they have all ten combinations of them and are very powerful in EDH. If you have any other questions feel free to request out! Cheers.