Anyone is permitted to purchase E+ on any flight. If the flights you choose don't have upgrade availability, you can "waitlist" for the upgrade. Pay for both Economy Plus (so you get your desired seat) and pay for the upgrade with miles and money. If you get upgraded, your E+ fee will be refund. If you don't get upgraded, you're miles and upgrade co-pay will be refunded.
Wait list upgrades can clear at any time and it is not uncommon for them to clear on the day of departure or even at the boarding gate.
Another option is to pay for the third E+ seat in the row. While you wouldn't have the lie-flat feature and will be served coach meals and beverages, the extra space between you can be an added benefit in addition to the legroom. If you chose to do this, it is best to call the res-center since there is a specific way it has to be booked. Just know that if you book the third seat and your upgrades do clear, you're probably not going to get a refund for the extra seat.