This is essentially like a roof on a house. The outer garment is there to protect the base layers from the elements so they they can function properly. Therefore, it seems logical, that if you have purchased good base layers which wick moisture away from the body you also have an efficient outer garment designed to let moisture out. Such materials as Gortex and Paramo's are expensive, but a coat could, in extreme instances, be a matter of life and death. If you restrict yourself to lowland walking then of course a high street purchase could fit the bill. Conventional waterproof barriers do resist the through flow of air from the skin. Evaporated perspiration will just condense on the inside of your coat.
Nowadays most hill walkers will opt for a breathable outer shell made of Gortex or similar and certainly over the last ten years the cost of these has fallen.  Design is also an issue and therefore a badly designed breathable jacket could in fact be less suitable better designed cheaper option.  Poorly sealed seams for example can lead to some jackets leaking - check for this when you buy a jacket.  Any unsealed seams will leak as each needle hole will encourage water to get inside.  Effective outer garments have well designed hoods, two way zips and an external map pocket.  In the shop, practice undoing zips, cuff fittings and opening and accessing pockets when wearing gloves.  If you are deadly serious try mittens!

Very few people would wear overtrouser all day.  As body heat is retained in the torso there is less concern when legs become wet.  Legs however can be protected.  When purchasing consider similar guidelines as to the coat.  You could pay £100 for overtrousers, but for the heaviest rain a cheaper semi-breathable would suffice.