Since you are going to read at night mostly so you may be asking for a back lit device and if it is so then forget about sony(if you don't wish to use lighted cover).
You will be left with Kindle PW, Kobo glo and nook glow. Not aware of any other lighted device.
No ereader is going to display pdfs in a better way they will be able to just display it. But Sony is better among those. Its also got a landscape mode option.
If you buy kindle then make sure you are able to buy books from other stores as well.
Sony is not cheap but will give you a better pdf viewing experience and trust be you will still not be satisfied if it's having pictures.
Kindles are cheap but with ads. and PW may not fit into your "as cheap as possible" criteria.
Be more picky as suggested by above members. And do watch video reviews they will help you a lot.
