It is a about a 4-5hr job (with the engine in situ) and more involved than a cambelt change but as with a lot of jobs it's having the right tools and experience if things go slightly wrong. Many local garages shy away from the job so make sure you get one who knows about this particular engine. A Vauxhall dealer will charge about £700 plus VAT for labour, then the cost of parts on top (so probably in excess of £1200).
It's prudent to renew the balance chain with a new kit at the same time as the timing chain kit. I certainly recommend fitting genuine GM kits, as aftermarket kits have proven unreliable, including timing & balance chain tensioner failures, chains not fitting correctly and stretching therefore requiring replacement a few thousand miles later. With genuine chain kits the job will be good and they last, after all that's what was fitted at factory and should last another 100k miles. The timing chain kit has undergone several revisions which includes the tensioner from the Z20NET and a modified oil jet to improve chain lubrication.
Genuine GM Timing Chain Kit (part # 55570337) - here
Genuine GM Balance Chain Kit (part # 55563405) - here
Hope that helps. 🙂