I have been down this road and I'd suggest considering the following.
Re the house itself - an acre of garden gets old pretty fast as a non gardener. You are looking at the herbaceous borders wondering what it all is and how to maintain it. Look beyond that, look at the hedging that will need trimming, the mature trees along the boundary that will rain autumnal foliage for 3 months and require raking. Look at the sheer scale of it and consider will your current lawnmower, electric [probably] hedge trimmer etc cut the mustard or wear out within months.
Our biggest lesson is that we've had to invest in public park scale equipment so a hundred pound rechargeable hedge trimmer has turned into a Stihl petrol one at three times the cost. We've had to buy new lawnmowers, strimmers, a long handled hedgetrimmer, chainsaw, leaf collection blower/sucker. So even if you radically simplify the garden you will need to step up your gardening game / employ a gardener / teach your kids and make it chores. We involved the kids when we first moved in and they were primary aged. Now that they are teenagers their only interest is in hanging out in the neglected trampoline when friends come around, the swing and throwing a ball for the dog. Persuading them to help in the garden requires cold hard cash. You may get more longevity with boys if they are keen on sports.
If you decide it's definitely worth it to get the house, say nothing until you have exchanged. Make a plan. A 10x50m rose garden sounds amazing and is possibly the most low maintenance thing there but if you want to turn it over to lawn, then simply tell the old owners that you have three young kids and will be doing that and would they like to take the roses or gift them to friends and neighbours. No reason why they can't come and take them after you've completed when it's autumn and more suitable time to do so. You can have beautiful flowers all summer and they can come and prune and dig them out. Lay the lawn in the autumn and it should be good for football by next spring.
A mature herbaceous border is something that a decent gardener can come in and blitz a couple of times a year. If the ground is covered, that will stop weeds coming through. So I would start with the areas you definitely want to turn over to grass and deal with those so there is a clear play area. After that, it's survival of the fittest in the plant space. But you won't need a weekly gardener. Quarterly will do but if you are buying a house this big, it costs more to maintain and that's just the size of it. As plants die off you can replace with something hardier, we have moved to a lot of drought proof plants as the summers have gotten drier and I can't justify gallons of water on the flowers. Water storage is a drop in the ocean in a large garden.
Also check the fences are in good condition. Expensive to replace if they are nearing end of life and the boundaries are yours to keep up.
Lastly and I'll be shot on here for this. Is there a potential to subdivide the plot so you keep a smaller garden ?