Marlow? For a prep in Oxford? It’s also quite distant from Aylesbury.
I think your best bet might be around Thame in Oxfordshire. Or villages nearby. Chinnor or Aston Rowant to the south or Garsington nearer to Oxford. Your big problem is traffic in Oxford if you live too far away. You will spend a long time in the car!
I live quite close to both places you mention for work but I would not want to commute into Oxford for school. So you will have to get nearer to Oxford. Brill is nice. You do have other preps in Bucks you could look at. Dorton House school is quite near Brill but it’s not The Dragon. My DD went to Godstowe in High Wycombe but they only do pre prep for boys. Many then go to Caldicott.
If you looked around Amersham, say Prestwood, Great Missenden, Little Missenden, and Whiteleaf, Speen, Lacey Green, Wendover etc you could look at The Beacon School in Amersham - boys prep. Lots of villages are ok for High Wycombe and Aylesbury. They are not ok for commuting a DC to Oxford.
Lots of nurseries here are fine. Several preps run nurseries. You might consider the Gateway prep at Great Missenden but it’s really an 11 plus crammer. Bucks has grammar schools. This is the big goal for many Bucks parents.
To get to Oxford, you need to be west or north west of Aylesbury really. And near the M40 for Wycombe.
I also suggest you need to think about your final destination for DS too. Grammar in Bucks or private. Boys private secondary day schools are largely absent in Bucks other than Stowe. So are you aiming at somewhere like St Edwards in Oxford or a Bucks grammar? Prep choice In mid Bucks is somewhat divided between grammar prep and independent school prep at 13. Let me know if you need any more suggestions.