You can buy the ticket at Staplehurst [[stn]SPU[/stn]] station ticket office, or the ticket machine, on the day of travel. If you prefer, you can buy it in advance at any station, or online, the price will be the same (unless you use a website that charges a booking fee, in which case it'll cost more!)
Simply ask for the Off Peak Return via High Wycombe.
There are no peak restrictions out of Stapenhurst on the Route High Wycombe ticket, though some staff who don't know how to look up restrictions (due to poor training) may think otherwise, in which case simply ask them to look up the restriction code.
The journey time is not much longer than from Euston; it's a lot faster than it used to be. Aim for one of the '
loco hauled' trains for extra comfort, if times suit you. All the trains from Marylebone to Birmingham go via High Wycombe, as long as the train goes via that route then the ticket is valid, they don't need to call there.