Dear Gulio,

A typical IX-c Uboat was 76,76 meters long. This means that if you're building it in 1:35 scale it should be 2,19 meters long and not 2,50 meters.
If you care to wait I should be able to provide you with accurate plans. That is when I recieve them myself and this will hopefully be in the near future.

To the other questions you asked in your posting. The only guns available as a modell suitable for the IX-c is the 2cm flak gun from www.Accurate-armour.com. Than there is a 2cm "Flakzwilling" from ADV/Azimut and a 37 mm automatic Flak also from ADV/Azimut. The 2 cm "Flakzwilling" gun is very hard to get and rather expensive because you need two of these when your building a late war type IX.(you should be able to scratchbuild two pairs from one Tamiya 2cm "Flakvierling" wich is still available)

For the big gun could buy a 8,8 cm gun from Accurate-Armour or Andrea and (ex)change the 8,8 cm muzzle to fit the 10,5 cm muzzle. The 8,8 and the 10,5 had almost the same features and the only thing that really was different(for the eye) was the muzzle. As for the old style 37 Flak gun. There is daunting task for all of us type IX lovers. That one you'll have to build yourself!!

I do not know what you mean by the 45 gun. Please enlighten me!
As for the book from Kohl and Niestle its still being printed try Amazon.usa or the used book section. If you're going to build the type IX-c that book is your bible!!.

Kind regards
Wiljan Bakers