Technical issues aside, I think as the car is an Opel there'd be a certain amount of charm attached to keeping it in the family and using the Vauxhall engine.
When you look at the technical nitty gritty the Rover V8 is a pretty ancient engine by modern standards but it is a very commonly used engine and therefore they are plenty of people who have trod the path before and plenty of parts available for tuning, etc.
You can also just slap a carb and a distributor on it and it'll run.
The Vauxhall engine has no provision for a distributor (quite rightly so for a modern engine) and I doubt you will find anyone making a manifold to fit a carb to it. So, your engine management is going to be more complex. However, I personally wouldn't build an engine using carbs or a conventional distributor these days. The benefits of mapped ignition and fuel injection are just too great and it's not that expensive or difficult to do. With care, you could probably re-use the original ECU but you'll have a lot of work to do keeping it happy when devices that it's used to controlling have gone. I'd probably go for an ECU you can map yourself, perhaps a Megasquirt or Emerald.
I find the Rover V8 is a very lazy feeling engine to drive, as well. Don't get me wrong, it's a powerful engine but whilst it generates huge torque from idle speed upwards it doesn't really thrive on revs, even when mildly tuned. It's got great characteristics for a heavy, cruising car but personally I prefer an engine that likes to rev a little more. I think the Vauxhall V6 would be a better bet in this respect.
If the Vauxhall engine will give you enough power and you aren't going to want to tune it much above standard then go for it. If you're going to want silly power you'd be better off dropping in a V8 that will produce it out of the crate IMO, or at least one that's well supported by suppliers of tuning bits.
Kevin