Vauxhall Astra loss of power and noisy tappets

When you get noisy hydraulic tappets it's usually a sign of lack of oil pressure and circulation , probably why your engine is now seized. Normal causes are wear and tear on engine or faulty oil pump . or both

That makes sense and can’t understand the mechanic not thinking about that,
Got cash to replace engine but need someone reliable and knows what they are doing, my Mrs doesn’t deserve being ripped off all the time when it comes to cars

You have every right to be careful and need to get a reputable , certified garage or engine rebuilder / remanufacturer . There are a lot of cowboys out there that will gladly part you with your hard earned cash and not think twice about it . Also make sure you get a good , full warranty whatever way you decide to go .