I know some guys asked irrelevant question.These guys instead of just giving their suggestions are just lost behind what other people think and they are after the life of people whose suggestions doesn't match with their suggestions/thinking.
Anyways don't want to dig out more things,but I am really sorry from their side.
So I hope you have got the answer.
But always keep in mind that a 17" laptop specially dell xps will weight a lot and will be difficult for your dad to carry around so don't forget to buy a trolley backpack for him with that.
And if you think that Windows 7 will take a lot of time to load data then I don't think you need to worry about that.Just get a lappy with i7 processor with good amount of ram as you said 8GB will be good enough( even dell gives an option for 12GB but I don't think it is necessary) and replace your hard disk with a Solid State Hard Disk and with win 7 it work flawlessly and you surely will not face any issue while loading any kind of thing/data