Your questions are by no means ignorant - they are very valid but by doing a search you will often find the same questions (and answers) come up time and time again.
I have the 170 bhp which gives similar performance solo and towing to my 140bhp 147 JTD that I had, albeit it seems a tad faster but not much in it.
The 136bhp actually has identical torque at the same RPM so unless you thrash the car regularly, you shouldn't notice much difference so don't discount it if the price is right!
Just bear in mind though you may always have it in your head that you wished you got the 170. Also take into account the resale - it might not be so easy to shift as a private sale.
There aren't actually any facelifts. We call them series models and the 159 had 3 series
. Bear in mind the following is a generic summary of the 159, not just the Ti.
Series 1 models had nastier looking 17 inch wheels and no blue and me and most models whilst the series 2 (like mine) had nice 17 inch wheels and blue and me standard on all models bar the Turismo and the series 3 had an interior change with black aluminium dash boards and [nasty] carbon fibre inserts (on some Ti models) instead of the nice silver brushed aluminium.
I understand that some Ti's had half leather an alcantara as standard with leather as an option (probably series 1 and 2???) whearas the series 3 had leather as standard.....the alcantara is the nicest though IMO.
The series 3 Ti also had red stitching on the seats and steering wheel.
The 59 plates will just about be available with the 2.0 but I doubt you would get an 09 with one. The 1.9 drives better IMO as it doesn't get so bogged down but then again the 2.0 is much smoother and when you rev it, it actually shifts the 159's mass quite well.
On top of that the 2.0 is cheaper on road tax, more fuel efficient (not by alot though) and apparently significantly less prone to DPF issues due to the fact that it is 'close coupled'. Not sure about the EGR though.
I loved the 1.9 in my 147 with the way it performed but it is an older design that was noiser and cost me a small fortune in EGR issues so I'm hoping the 2.0 is a step in the right direction
I hope this answers some of your questions.