My advice is to ask for pictures of previous work. Most good trades people have a website or page where you can view work. I encourage clients to look at my facebook page where I post pictures of jobs I have done. Plastering is a really skilled job so I'd be cautious if you get a really cheep quote, I'm not saying you should pay through the nose but just be on the look out for that. If you want to sound like you know a little about the job ask them if they will be sealing the wall first (if its a re-skim). This should be done using PVA to seal the wall first. And please be cautious about people who repeatedly bang on about a mirror finish. This drives me mad, a mirror finish isn't always the sign of a good plasterer.
I hope this helps.

Hi, I would defiantly ask for pictures of previous work or even better if they have a job you can come see, any good plasterer would be more than happy to have you come see their work. In regards to stopping all cracks if the work is over plasterboard you shouldn't have any cracks appearing, although if it is over an old background even with scrim taping the cracks you can never guarantee they won't re-appear. A good plasterer would explain this when having a look at the works required and recommend best options.