You are very young to be considering an equity release - only just eligible for some, and too young for most where the minimum age is 60 or even 65.
If you sell your home to a reversion company, you will only get 30 - 60% of the market value. The older you are the more you get - as the company doesn't have to wait so long for you to die.
Another way is a lifetime mortgage - you get a lump sum but make monthly repayments as interest. The company gets the capital when you die. As you are so young, you could be making these payments for 40 years.
In a nutshell to your questions :
You will not get full market value
When you and your wife die the house must be sold to repay the companies. Your sons could not rent it.
In depth information here:
http://www.unbiased.co.uk/find-an-ifa/guides/m ortgages/equity-release-unlocking-money-from-y our-home/