As you've found out, High Wycombe is the closest but of no use to you as they only have a MLU so you can only go there after 37 weeks. They don't do monitoring or anything either so you'll have to go to Stoke Mandeville (Aylesbury), Wexham Park (Slough), John Radcliffe (Oxford) or, possibly, Royal Berks (Reading). The first three are about equidistant (in terms of time to get there if not actual miles) and, in my experience, are the ones routinely offered to pregnant women in Marlow. Royal Berks is further away and isn't usually given as a choice.
The majority of women in Marlow who I know (DC1 is 2.8 & DC2 5 weeks so I know a lot of women who have had babies in the past three years or are about to) go to Stoke Mandeville for the simple reason that all of the ante-natal appointments can be done at High Wycombe so they're a lot easier to get to. It has a MLU and a CLU and also has a neo natal unit. It was refurbished about three years ago & still seems quite clean & shiny. I had two very different births there (DC1 was induced, epidural, forceps in theatre; DC2 was natural, only there for 30 mins (gave birth less than 2 hours after my first contraction), G&A) but the care for both was good. The food on the post-natal ward is atrocious so, if you do end up there, pick up some food on your way! Also, if you do go there then, from Marlow, don't follow the directions to Aylesbury as SM is outside Aylesbury. Instead, take the A4010 through High Wycombe in the direction of Princes Risborough & follow the directions for Aylesbury and then the hospital from there. The entrance to the maternity ward is quite hard to find as its tucked down the side of a porter cabin. The journey would, typically, take 30 - 40 minutes as the roads are mainly single lane A roads and go through some small towns & villages so, in places there are 30 & 40 mph speed limits. When I was in labour, we managed to get there in 20 mins although this was outside rush hour.
Wexham Park does not have a good reputation locally although 2 friends have given birth there in the past year & didn't have any complaints. I don't know much about it as we never considered it.
John Radcliffe is a university teaching hospital with a CLU, MLU and a neo-natal unit. Two medic friends who had ELCS (for medical reasons) chose the JR over hospitals which were closer to them. However, other friends who have had straightforward pregnancies & delivered there have commented on how busy was, how rushed any post-natal care was etc. To get there, you take the M40 then the A34. Both of these can get very congested at rush hour but otherwise its mainly motorway & dual carriageway so you could get there pretty quickly if you needed to.
As for the maternity/baby shops in Marlow, they've all closed in the last 12 months or so leaving only JoJo Maman and the one in Marlow doesn't have any maternity stuff.
I hope you have a straightforward visit! Marlow is lovely.