The language is old and it wasn't standardized in spelling, as all the "shews" demonstrate. I think the first attempts at dictionaries trying to standardize it were only about 100 years old, but the most successful attempt by Samuel Johnson was contemporary with Hume.

I like Hume a lot, but I wouldn't say it was easy reading and you have to be pretty familiar with Descartes and Berkeley to understand what he's talking about and neither of those are that easy. Descartes has the benefit that it gets translated into English about once a generation, which makes it easier to read.

As far as his English history goes, it's supposed to be very good for the time, but at this point we have such better methods and so much more research to rely upon, and greater access to archives that have been better maintained that I think the only people who really read it anymore are academics studying Hume or the historiography of English history.

I would definitely make an effort to read Hume's Dissertation "Of the Passions" and his two Enquiries. It's better if you can read them as part of a class with an instructor who's taught them before b/c there's lots of context. Reading Adam Smith's Theory of Moral Sentiments helps too. But it's a lot to read and none of them are easy going. But it's very rewarding.