Do you want to go safe and farm? Or actually be useful in the early through mid?
A good middle ground if you aren't sure is just building attack speed. Maelstrom into Mjolnir is good farm, good push, good damage, and large teamfight presence all in one item. Follow it up with AC and you are near unkillable and have the attack speed and armor penetration to explode enemies.
"Just let me tell you a little lesson buddy. The cream always rises to the top...And I'm about to show you the white hot cream, of an 8th grade boy!"
He can be a decent support, initiator, carry, tank, etc. He's not just a dps machine.
For DPS Alche I'd skip Midas. I go Shadow Blade, Mjollnir, and then it depends on the other teams lineup. AC after that can be fun for crackhead attack speed. Basher/abyssal blade is good.
Midas is really unnecessary on him. Your farm should be so out of control anyways and carry Alche's advantage is that he gets 6 slotted by mid game even without Midas. He's made a decent carry by the fact that he gets online so much faster than anyone else. Getting midas just prolongs that. Your going to get 6 slotted by late game regardless of midas and you're going to get there faster than anyone else. You need to dominate the mid game as Alche is weak late game compared to other carries. You should be destroying the other team mid game not farming till late game where Alche is weak.
It's also bad considering you are probably going to get Mjollnir. An item that makes your farming godlike anyways. Midas is overkill. If you get Midas you might as well get Bfury too for the lolz.
Shadow Blade allows Alche to farm without fear of dying as it essentially makes you un-killable even if they counter with dust as you will still have your ulti. You can push/farm/gank with impunity. On top of that you become a much more dangerous hero ganking wise as you can land your stun easily.
Mjollnir makes your farm absurd without sacrificing dps like Midas. It's also a decent late game item unlike Midas and the active works well with Alche.
As Alche you need to end the game early.
Your skill build depends entirely on how the lane is going.