"a lot of my downloaded books are PDFs"
PDFs scanned from books are what ereaders are worst at viewing. The PDF images are too small to be comfortably read on the 6" screen that is most common for ereaders; and because the PDFs are OCRs of hard copies, they are graphic images and therefore don't reflow under magnification, and their processors aren't fast enough to make chasing a magnified image around their screen a comfortable experience.
There are conversion tools out there but your converted results will be in direct relation to the professionalism of the tools and your dedication and expertise.
On the positive side I have recently noticed that a lot of the PDFs of public domain books are being joined by epub versions of the same texts - a far better solution for you (and me).
Your list of needs pretty much tells you that you don't need 3G or WIFI as those options are for downloading popular fiction from the captive retailer who markets whatever brand ereader you purchase.
A good epub/"good" PDF format reader is probably what you want. The Nook and the Sony are both good with epub, and both can reflow PDFs that are not graphic images (scans).
If you are looking at used kit, the PRS 505 is widely praised for its screen quality and is inexpensive on the used market, but keep in mind that the battery may not hold an optimum charge anymore and cannot be replaced by the end user so you may end up with the additional expense of sending it in for a new battery.
The Sony Daily Edition is $250, I think, but it has a larger screen and "might" be suitable for scanned PDFs.
If you are interested in the opinions of the hardcore and early adopters try reading this forum:
http://www.mobileread.com.
Regards,