You need to do a budget for the lifestyle with the house as well. It's not just the mortgage you need to factor but the cost of heating it, council tax, home insurance, etc.
We moved from a 3 bed semi to a 5 bed detached house. Due to equity we had built up in the first house there wasn't much difference in the mortgage £600 to £750 (fixed 5y so not at current rates yet) but council tax is £100 a more, heating is £60 a month more, home insurance is more - you would need buildings insurance as well as contents
We have life insurance that will pay off the mortgage that is £40 a month for both of us (one policy each)
You want to consider critical illness and income protection cover too.
Once you have all of those costs, add in decent pension planning - how much work would you both have to do and how to afford this house and lifestyle, then as part of your 65+ plan what would you need to be able to afford this as you reach the age where some people are thinking about slowing down and approaching retirement age.
We worked out that on our salaries there wasn't a benefit to critical illness and income protection as our lifestyle was more than covered by either our wages and we worked out that we could also afford it on both of us on lower salaries. We were therefore able to save quite a lot when we had two wages and have a considerable buffer. Now we are on one salary and not had to adjust our lifestyle