Hi Bev,
If you wanted to buy a business, you would have to apply for a Long Term Business Visa. These have many points you have to match before you can qualify for one. Firstly, you need to have business management/ownership experience, a good sound business plan plus a bit of spare cash behind you to support yourselves for the term of the visa (on the assumption that your business will not make money)

Strict guidlines to follow, and generally not an easy route.

As for your parents retiring, they will need to contact the local immigration office as retirement is a bit of a case by case subject!

If you want to know more about the LTBV, let me know and I will help as much as I can.

All the best