For emails and internet browsing you don't need an expensive machine.
First thing you need to decide is the size. They are usually 7" or 10" (there are a few inbetweenees) and I decided on the 7" because it is much easier to slip in to a pocket or bag and very easy to hold with one hand.
I read all the reviews and decided I wanted an android machine because it would sync easily with the Chrome browser on my desktop and has a huge range of apps.
I didn't want the cheapest of the bunch as the screens aren't so good and the processors aren't so fast.
I settled on the Nexus 7 at £200 and have not been disappointed. I use it as a satnav in my car (much better than my TomTom) as well as browsing the internet, emailing etc. I can download free library books and use it as an eReader (I can also buy books and download free books from other sources), I can listen to the radio, watch tv or films.
The weather and news apps are particularly good.
A tablet is designed to be 'always on', the same as a mobile phone, so it is great for quickly getting information instead of booting up your pc.