Hi Wilco,
I just got my PPL in June.
Do Class One medical first and then forget about medical for five years unless you go commercial.
If you can, get all the exams done as soon as possible. They will all help your flying.
If you are used to studying and fairly bright you'll have no problem. They are not really heavy and you can get through them quite quickly if you apply yourself. The content for each individual exam is small enough to be able to learn everything for that exam fairly quickly, revise it, test test test yourself and then sit the exam straight away. Little break, night out, and move onto the next one.
I don't think order is that important but, my humble thoughts:
Do Air Law early cause they'll want you to before going solo. My instructor got me to sign a bit of paper one day and was about to hop out the plane and unexpectedly send me solo - I hadn't yet done Air Law - so no go and weather prevented actual first solo for another month or more - pretty annoying.
Principles of flight & Flight Flight planning - principles of flight probably a good thing to understand when doing early flight training especially stalling, and good to think about weights, balances & planning early on so you are at ease with it when it comes to flying navs - perhaps do this one early on.
Navigation - pretty substantial one and alot of stuff you want to know and be good at for actual flying rather then just to pass the test. True that it helps to be doing navs to put it on context so a mid-way one maybe. When practicing, be silly with your accuracy - squint with your eyes up against your protractor when measuring angles, draw lines from the exact middle of the little circle that is an airfield on the map, very precise with whizz wheel etc- The exam requires you to be very accurate with your angles and distances - a small mistake gets amplified by subsequent calculations.
Meteorlogy - Depends how scientifically minded you are - quite substantial. Can probably leave till later in the course as you will probably rely on your instructor to make the weather decisions until later navs. But you should start looking at the weather everyday, looking at TAFs, METARS, and briefing charts and get used to thinking about whether you would fly into it - develop your judgement.
Aircraft General Knowledge - pretty basic mechanics, electrics etc. - just read, memorise & test.
Human Factors - pretty basic human biology. Again just read, memorise and test test test.
RT - The longer you've been flying and using the radio the easier this is going to be and no need to do it until the end so do it near the end. Make sure you are doing all the rdaio work from as early on as possible.
You have to pass the last exam no later than 18 months after passing your first exam.
Again, if you can, get all the exams done as soon as possible.
I hope this is of some help. Do let me know if you have any questions at all about exams, material, flying etc. etc. etc.
Cheers
Matt