put it this way...
the mechanic I use, took the head off a zafira last week, and replaced all piston rings and rebuilt it in 6hours.
I (and I'm certainly NOT a mechanic

) could take the head off of my valver and replace the manifold in a couple of hours.
Saying that it would be trickier if some studs snap but still easily "doable" yourself.
Just get everything you need and might need of things go wrong, and crack on, you have 14days AFAIK to get it retested for free.