1) A nuclear device

You can't buy nuclear or other WMDs for a hundred Galleons. And the instruction implied buying, not to stealing (and what wizard capable of stealing nuclear warheads would want or need such a paltry payment?)

More to the point, Harry - or any wizard - can just Transfigure WMDs. Quick, free, and no need to carry dangerous stuff around.

He can also Transfigure antimatter. If he can come up with a reliable remote trigger, that is gram per gram the deadliest thing known to Muggle science. It was also foreshadowed a bit (chapter 14):

Say, Professor McGonagall, did you know that time-reversed ordinary matter looks just like antimatter? Why yes it does! Did you know that one kilogram of antimatter encountering one kilogram of matter will annihilate in an explosion equivalent to 43 million tons of TNT? Do you realise that I myself weigh 41 kilograms and that the resulting blast would leave A GIANT SMOKING CRATER WHERE THERE USED TO BE SCOTLAND?

2) Some other form of muggle weapon

As above for any weapon I can think of. Antimatter and nuclear weapons trump any area-effect attack. Poisons and biological or chemical agents can be Transfigured too, unless you need them to last for a long time before acting. Magic trumps Muggle personal weapons. ETA: not Harry's magic, though, and guns can be useful against weak or unprepared wizards. Electronics, computers, and software are Sirs Not Appearing In This Fanfic.

Ingredients for a magical ritual/potion which will resurrect Hermione if the main plan fails

Interesting. An irreplaceable Muggle artifact that was created by expending life in some manner, so sacrificing it would release "life energy"? I think though that defeating Death in such a purely magical would not be appropriate for Harry.