They're leaving the franchise. The number of new units quoted would be larger if they stayed. Anyway it would take more than a loo to make them suitable for main line running.
The number of new trains that First can claim surely must only be new trains that they have ordered, especially as some of the 707's should be in traffic by the time the franchise changes over.
The 30 x 707's plus the 90 new (First) trains would give you a total of 120 units, however if we assume that the 90 new trains are formed of 60 x 10 and 30 x 5 that would be the equivalent of 150 units with 5 coaches. That compares with 157 units with 4 or 5 coaches (excluding the 444's and 450's) which would be what the franchise will have as of the end of 2017. By having two fleets rather than three (including the 456's with the logistical headache that they could cause when trying to run 10 coach trains) there is probably some savings to be made in the total number of units required.
However with the 422's coming back they could allow 450's to supplement the services run by the older stock (i.e. Ascot - Guildford services) to make up some of the shortfall.
With the 707's being capable of 100mph running, if we assume the new trains are as well, then that could be where a lot of the time savings in journey times for the metro services are going to be found (it could also reduce the number of units required to run the same level of service).
I could be wrong but I think that there is a chance that the 707's are going to stay and it's the older units that are the ones moving on. From the above it certainly implies that to me rather than the 707's being the ones being moved on.