What you can borrow is based on what you can afford. Each mortgage provider has different affordability criteria. Your advisor will complete a full income and expenditure assessment to help you establish how much you can borrow.
Some mortgage providers will accept benefit income. As most means tested benefits aren't guaranteed and can change most mortgage providers will want to make sure you can afford the payments should your benefits reduce or stop.
You can still qualify for a mortgage although it will depend on your new salary, type of contract and any probationary period.
Some mortgage providers will allow you to maintain a mortgage in retirement providing you can afford the payments, though it would need to be repaid at a specific age. If you're in or near retirement you can also consider your suitability for a retirement mortgage or equity release plan.