Vems - does your flying club have a decent selection of sets? Thats how I found out about the DC 13.4 - by trying it one day in a lesson. GIZZAJOB is quite right that a badly fitting headset is a nightmare and the last thing you want is to own said headset.
If you can't try many then my advise would be to read reviews online and have a look at the padding at the top of the headset, though there really is no substitute for trying them I suppose (GIZZAJOB hates DCs and I love them so who knows what you'll think).
In terms of the order of exams my instructor told me Air Law, Navigation and then it doesn't really matter (though I'd say Comm third but then thats something that comes with practice). Do you have the PPL Confuser? If so thats really useful for exam prep, especially in something like Air Law.