Rush RoA, then judge your opponents anti-heal ability ie. Champions with anti-heal abilities as well as ignites and amount of them. If they just have a Champion with anti-heal or just 1 or 2 ignites then you should get Wota or Spirit Visage next. If they have Champions with anti-heal and ignites then you will need both. Judge their AP abilities as well as to whether you need SV or WotA as the SV Mres can mean a lot to survival.
HP, Mana Pool, anti-heal survival, AP, and CDR are the thoughts you want when building Swain. RoA gives the first 2. WotA and SV give the last 3. After all that Rylai's, Deathcap, and Zhonya's are generally going to give you the most on the AP survival side of the house. I do get a Thornmail occasionally to counter a team with 2 or more AD heavy hitters. Lastly Mpen in runes and only get an item with Mpen when the other issues have already been covered. An Mpen item is nice You just are not likely to get as much out of it unless your opponents are stacking a lot of it.
Of course boots of choice. Usually merc treads or sorc boots depending on opponents cc. I usually will not get t2 boots till mid game. Mobility is not necessarily needed for Swain as this build is kind of an AP tank build.