The transfer of 150s from
LM▸ was never intended to provide a significant increase in
future capacity - as any cascaded units arrive, other temporary short term fixes such as the hauled stock, the
ATW▸ 150s, the Northern 142s and last but not least the
SWT▸ 158, will all leave, job done. The only real gain is in better diagramming of a more consistent fleet, in other words higher utilisation. There is a capacity increase over the situation 3 years ago maybe, but not really over the current state, IYSWIM...
TOCs do not buy stock - the 11 x 4 car 172s would have been leased from a Rosco as normal.
I reckon your 2014 is far too early - add at least a couple of years. The first Thameslink stock won't be delivered until some time in 2015, and even then I'd expect the first dozen or so trains to be replaced will probably be the 23 x 377/5s sub leased from Southern.
The Siemens-led venture will deliver the first new train on to the network by the start of 2015, with the order complete by the middle of 2017.
Paul