According to today's announcement, electrification will be completed by 2018, so any decision as to whether to use cascaded stock or to order new trains will have to be made in the next couple of years.
If the decision is made to use existing stock, 315s used on Liverpool Street to Shenfield stoppers will be cascaded to ATW. Since there won't be enough 315s to replace the 15 142s, 15 143s ATW has, and however many 150s are used on Valley Lines services, it would make sense for whoever wins the Greater Anglia franchise in 2014 to order another batch of electrostars, meaning that all 315s can be cascaded to ATW, since 317s can be cascaded to Liverpool Street - Chingford and other routes worked by these units. Then, all Pacers used on Valley Lines services can be withdrawn and 15 150s can be cascaded to whoever needs them most, which will almost certainly be Northern, since by 2018 the Pacers will be one year away from compulsory withdrawal.
If the decision is made to use new stock, the stock Bombardier will tender will probably be identical versions of whatever trains it proposes for Crossrail. Hopefully the decision will be made to lengthen platforms where appropriate so Valley Lines trains can run in eight-car formations.