Id find the nearest specialist, and spend the 600 + to get it fixed... shouldn't be 300 on top seeing as they going in anyway surely, id be toying with a water pump too just so i could sleep.
How much would it be for just a new head... id write off your head (would then minus the drilling out time prolly saving 110... ) Aquire a new head, go to specialist and have the heads swapped before anyone does more damage boring metal and risking it.
It would of broken up if you did it your self... probably... theres only really one tool to take one out and even a tool of a person can remove a spark plug with it... when i go in i tend to give them a little tighten before i undo them.. just a tinny nudge they all come out sweet after that.
Oh and only take them out when they are cold so they haven't expanded tighter.
Do what you can yourself... if you screw it up its a learning curve... new head either way... If you think its worth it.
Next time do your own plugs... easy job
What details of car? 10v 2.4jtd... is that the 150bhp? one?