Welcome to MobileRead, dnoh.
Unfortunately, you're not going to like the answer to your question:
none of the current devices do a very good job of reading A4/Letter sized PDF files.
They do just fine if the "pages" in the file are sized to them, but when you shrink something down from ~13' diagonal to 6" diagonal ... it's just not legible.
The Sony Reader (not "EBook" by the way) won't work for what you're wanting. The closest device to doing what you're after is the iRex iLiad with its 8" display, but it's twice your specified price range, and still requires some compromises.
The problem is that PDF preserves page layout above everything, so it just won't handle displaying on a screen below a certain size, unless there's a good way to zoom and pan, which present e-ink refreshes too slowly to do.
The only real suggestion I can offer is that you look into one of the smaller laptops or tablet PCs, a 10" display on one of those would work fairly well for you, but the trade-off there is the battery life, of course.