Hey-ho! Thisfox is right on all accounts. Let me elaborate a bit more.
1) The standard ASCII graphics are a tileset too. To my knowledge, there are no performance concerns regarding tilesets (except maybe high-resolution?). The only problem to remain wary of is "pigeon holes", or in other words, re-used graphics. For example, the "arrow pointing up" ↑ is used as an arrow per se, but also as a tree icon. If your tileset changes the arrow to look like a tree, then the parts of the game where an actual up-arrow is supposed to be, will look weird.
There are ways to mitigate this kind of problem, but ultimatively there is no perfect solution. I recommend you simply try out a few tilesets and pick one that looks good to you. Plenty of those available online, take a look. Personally, I stitched several different tiles together into a custom set!
2) Your observations are correct. Walking never exhausts dwarves, even if they're walking up stairs or ramps. In fact, it is generally agreed upon that fortresses spanning several z-levels are more efficient due to interconnection and thereby shorter travel distance between any two rooms.
(Minor caveat: Diagonal movement takes slightly longer.)
3) When tapping into bodies of hazardous liquids, the golden rule is to place a door or hatch somewhere along the tunnel, just in case something goes wrong and a flood ushers into the fortress. The loss of a miner is misfortunate, but the loss of an entire fortress is terrible (or Fun if you have an active sense of humour).
Mine from the target location towards the source, and finish with a channel from one z-level above the liquid. This way, the miner will be standing above the liquid and safely* watch it flow by.
Sideview:
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*(Except if the freshly dug channel is part of the way back home. In that case, they will deliberately walk down into the waterway before it fills up, and then may get caught in the current.)
Water is generally more forgiving, since dwarves can survive short "drowning" and can stand in shallow water.