Hi Andy,
1) I really dont know but undoubtedly very powerful. Probably in the range of hundreds of watts. Also if the stainless is nice and polished it may reflect the beam with bad results for your health.
2) Not that Im aware of but there's probably some suppliers to the medical professions.
3) If your looking to get your lasers in the UK they're going to be quite expensive as they will have to conform to lots of regulations. Also, powerful lasers such as what you would want to cut stainless with are regulated and you might need some sort of license or permission, to buy, own, use, and or import one.
4) As inexperienced as I am, Im going to go on a limb here, and say basically....no. You would have to constantly re-adjust the mirrors as the router application would probably slightly move them. Also there are completely different parts on a laser that need to be in place and adjusted and safe from getting damaged etc etc. I dont think this is a viable option.
People have adapted a router to take a quick (relatively) changeover to plasma cutting. Maybe this would be an option for you to look into.
These replies are just my opinion and may be proved wrong by someone else.