I think there are a few exceptions but generally you cannot purchase a preferred main cabin seat. Preferred main cabin seats are not upsold or upgradeable like Comfort+ but rather are reserved for those who are paying full coach fares (Okay, I guess you can purchase it) or those that have Medallion status.
Also, sometimes Comfort+ is a lot cheaper to purchase later on. On our next trip to WDW, it was $250 roundtrip in main cabin or listed as $350 for Comfort+. Well, after we booked -- Comfort+ was only an extra $40 roundtrip so I did it. Granted, it's just a 1 hour flight.