Unless severely overheated, I doubt that's your problem. There are also tools, tests that can positively identify a real headgasket problem so pay to have it tested before jumping to probably a false conclusion.
To answer your question, it's a very costly and time consuming job. Special tools are also required to do it correctly so it's not really a DIY job for most people w/o experience working on those engines.
I'd highly advise paying a dealer Volvo master tech to properly diagnose the problem(s) before guessing at other things that may not fix the real problem and cost you alot of money.