Facelift cars only have three diesel options... 1.9 120 8v, 1.9 150 16v and 3.0 190 v6
Not going to get into an argument about specs etc, but I know that I am right for 2006 on as I was looking at new Vectras at the time and the Signum was bundled in with the Vectra in the brochures... Which I binned when I cleared my room out last year.
My experience of the 1.9 150 16v lump in both manual (30k) and auto (15k) guises was less than positive, hence all the Vectra Cs I have subsequently owned were deliberately 1.8 petrols which I will defend to the end of days.
The Signum is a Vectra C estate with a funky rear seat and slightly smaller boot.
For the needs you suggest, facelift will be perfect, especially in Design or Elite specs and with whichever engine you can find looks best in Moonstone grey metallic or Reubens Red
Shop around and buy on condition regardless of engine
Incidentally all have 100k cambelt, except the V6* which are chain driven
*I know the 3.2 is belt driven, but early cars only, and I have nowhere suggested buying a preface lift Vectra C of any shape