1. What would be the best evolution of Evee?

2 In the resort area, or that other place, there is a house to the left of the Pokemon center. Go into it. Talk to buck. Go back to stark mountain. Get to the end. Battle heatran. He is level seventy

1 i would say vaporeon, because it has a buttload of health, and is not weak on too many types.

hope it helped

1.For sweeping purposes, Espeon or Jolteon.
2.Buck lives in a house in the Survival Area. Visit and talk to him there and he will return the Magma Stone.

1. It depends on your taste and how you battle. No Eevee evolution is best. If you want high Attack, Pick Flareon or Leafeon. If you want special to count, choose Espeon or Glaceon. If you want one for speed, choose Jolteon. If you want high HP and a decent stat balance for an Eevee, go with Vaporeon. Use Umbreon if you want high defenses. 2. Go to the Survival area and talk to Buck's Gramdpa. (He is in one of the houses.) Buck will return the stone. Head back to the room which he swiped the stone to encounter a Lv. 70 Heatran.

1. Vaporeon, because all he needs to learn is Ice Beam and he can take down his big weaknesses, in fact, almost all of them. Jolteon is good for speed. Flareon and Leafeon are good for attack. Glaceon and Espeon are good for special attack, and Umbreon is good for special defense.
It just depends on your taste for pokemon.

2. You go in his house next to the Pokemon Center in the Survival Area and he'll go "return" the Magma Stone. But if you go back to the same room, a level 70 male/female Heatran will be there, now.

I would consider breeding 5 evees and getting all the stones required and get 1 of every form.