I realize people really don't like Archangel's for some reason, but I almost always make that my first buy, choosing Tear or Blasting Wand as the first comp depending on whether I really need the extra AP early. After finishing Sorc Boots it's usually either Rylai's or Haunting Guise (the latter if squishies aren't building any MR). I don't like RoA on him as much as I feel he's less dependent on the survivability mids like Swain need RoA for. With the crazy mana/5 AS affords it pretty much allows me to spam spells as quickly as they come off cooldown if necessary. From there it's either Deathcap or Banshee's Veil/Quicksilver Sash, depending how the game's going.