I'd say that if you have the opportunity to do an EPQ, do it. It will enhance your CV and universities will like it - especially if it relates to your chosen area of study. However, if you don't do an EPQ universities will not judge you, as many schools and colleges don't offer it.
For the EPQ, you can either do a 5000 word essay or an artefact (performance or object), but at least a third of the marks come from planning the piece and talking about it afterwards. Do not be daunted: you will have the same amount of time as an AS to complete it, and after you write your dissertation, essay or thesis, 5000 words really won't seem like long enough.
For a science-orientated EPQ, people often write theses based on experiments (I once saw an interesting one where someone was messing around with penicillin) or essays (on stuff like the privatisation of the NHS or the benefits of some random health stuff), but really you can write about anything - one friend who takes sociology, chemistry, biology and geology did how to get away with murder, and another friend who takes languages and textiles just translated something old and wade it into a wall tapestry.
You start off learning how to research, you research, then you write it and fill out your log book as you go, about what you are doing.
As for subject choices, all you need for medicine is chemistry and one other science (in this case biology or computer science). Also note that favoured subjects by universities include chem, bio, eng lit and Spanish, but not computer science. And, whilst we are advancing in the technological age and computer science may help you, Spanish may be useful to communicate with future patients and languages are shown to be looked upon positively by employers. In he end though, it's down to what you enjoy more (you could always to what I did: take the subject with the most people and then if you don't like it, drop out and swap to the other one which just so happens to have some places left...).
Anyway, good luck, whatever you choose!