You don’t want to do gardening and you are concerned about the work, yet you want to rip out a rose garden and replace it with vegetables? I promise you, the veg garden will be far more work, believe me, I know.
Is this the property for you, and not a large, new property with a small, plain garden on an executive estate that would suit a growing family?
I wouldn’t underestimate the work and cost of demolishing the garden, having a planner in, even out the beds etc, laying more lawn, hiring equipment etc etc and if you are not keen gardeners, it may well look a complete mess if try it yourselves. My former neighbours did this when they moved in, due to having kids and dogs. Twice they sectioned the lawn to reseed it, so it was unusable as the kids were kept off it. Always looked a shabby mess. They stayed at the property less than 5 years. Where will you be in that time?
As another person has said, it probably wouldn’t require the upkeep you think, if you are prepared to be a bit more relaxed about it, and certainly I suspect it would likely look better than any change you would make if you are not keen gardeners. These days my husband and I can’t manage the garden how we used to. We are more relaxed, just sit and enjoy it, but every now and then put in a couple of hours and make a big difference in that time.
Obviously it’s up to you, but the song “They paved paradise, put up a parking lot” comes to mind, (though it’s a bit too harsh), but I also fear for the bees, butterflies, birds etc that will have benefitted from a garden like that, all of which also teach your children to value biodiversity. I hope whoever buys the property will really cherish it. This is the other point though isn’t it: someone will buy it, who knows what they will do. If you care deep down, you might be the best people to buy it.
I agree with the suggestions that if you purchase it, don’t let the plants go to waste. Contact your local gardening groups AND hardy plant associations and ask if people want to come and dig up plants from your garden. Ensure you give them time to inform members of their clubs at their meetings so confirm with them when that will be. Also offer free plants to neighbours. Other local FB groups would be worth investigating for ideas of who would like them.