Help! Advice for a complete novice

hey. Just brought my 1st clipper cabriolet. Its currently sitting on some really awful 13"s, I definately want to lower it but dont know how much. Been told to use 40ml springs and put 15"s on it and also been told to lower it 60ml.
You'll have to bear with me as i'm stil learning but surely 60ml would be a bit extreme? As the islands roads are really bad I dont want it scrubbing over every bump!
Im assuming 15" alloys would look better than 17"s but how much can I lower it by?

oh and I forgot sorry………..
will mk2 springs fit a mk 1 or can you only use mk1?

Not 100% certain but don't think MK2 will fit