I made reference in my posting to the statement that "the Government were sounding out manufacturers on the possibility of a DMU order", so that indeed would appear to be some political input into the equation as you state above.
The Government are being seen to do something (without committing to anything) - politics.
Yes agreed.
The DfT will need to see if manufactures are will to produce a product for the UK, the life-cycle of the product, compliance, costs, and time-scales to delivery of design/product etc. I expect that there will be a premium to pay.
However, at the end of the day, it will be a politically motivated decision as to whether a procurement will go ahead. The roll-stock plan, produced by the operators, indicates that there is no near-term requirement for DMUs, an only the possibility of a requirement later on. Spending money on vote-buying will take the funds from elsewhere on the railway.
True.
I appreciate that politicians have to be seen to "keep all options open" and at least give stressed commuters the impression that they aren't automatically ruling out brand new DMUs, but the reality is that it'd be incredibly expensive to procure new DMUs to enter service in around 2018 when we'll have maybe a couple of hundred surplus DMUs by the end of this decade (with the prospect of more to become surplus in CP6).
Paying a premium for having spare DMUs for a couple of years (and so that we can withdraw the final handful of Pacers a year or two earlier) hasn't been ruled out (yet), but as you say, it'd be an expensive option in the long term.
With lots in the press lately about devolved powers for the nations and regions, the press need to kick up a stink about the difference between the new Scotrail franchise and the proposed Northern franchise.
Scotrail get HSTs, brand new EMUs, lots more capacity, tourist coaches etc, Northern get no prospect of anything positive except being made to do more for less.
As a Scot living in northern England, I'm not particularly envious of the new ScotRail franchise.
Replacing the 21st century 170s with four coach 1970s HSTs doesn't seem like a huge leap forward (look at all the frothing about the age of the 1980s 319s coming to Northern!).
Plus some of the additional EMU capacity in Scotland is coming in the form of Class 321s from London (again, imagine the wailing about "ancient London cast-offs" on this Forum if the 321s were due in Manchester

)
The much hyped "tourist trains" appear to be nothing more exotic than 158s with some seats taken out (plus, whilst it's going to be part of a separate franchise, Scotland will also see 21st century Class 67s on the sleeper replaced by 1960s Class 73s - grass isn't always greener)
Don't get me wrong, there's some positives with the new ScotRail franchise (as there are in virtually all franchise announcements) - I just don't think that it lives up to the hype.