If the train builder delivers two trains every three weeks, it needs 135 weeks. Allowing a couple of weeks for fault-free testing of the final deliveries, that means delivery should start early in May 2018. They've got to set up a supply chain first. Relatively easy if they are already producing a similar train, e.g. Aventra, though then the capacity of their factory and of their suppliers' factories may become an issue. But it's clearly possible.
If deliveries were one per week the first one needn't arrive till early in 2019.
There's a big advantage in buying a train which already has approval for UK use.
There's a lot of work First/MTR will need to undertake. Driver training, maintenance training, depot modification for the new stock (they need to be able to empty those toilets), driver schedules for test-running. Storing the new trains before they enter service, storing the old ones before they depart to wherever they're going.
I've no doubt that it can all be done if everything goes to plan, and the computers at First/MTR HQ will already be hot with Microsoft Project (or whatever they use), but there are also a lot of things that might intervene.