welcome to the site
theres not masses between the 4 and the 5 other than interior and exterior trims and shape, engine wise theyre pretty similar.
i'm a bug fan too, owned loads including a newbie and even now still go to aircooled meets
what to look for, well if your getting it from a dealer you'll have warranty. but look for wear on the seat bolsters, scuff damage to the underside of the front bumper, the fans as youve mentioned, check the side skirts - that they bonded on well and basically the paint
other than that theyre pretty bullitproof you'll find so its just common sense