Hello pauluscaesar and welcome to mobileread!
Ill try to answer your questions.
Every reader reads only one drm format, but multiple non-drm ones. If your files have no drm, chances are you can read them just as well on any reader, or if not, you can easily convert them.
3) touch or not touch screen
This is a matter of personal preference. Readers with dictionary support and search need some kind of keyboard, so they either have a hardware keyboard like the kindle or a virtual one like the sony prs-600. For that, you need a touchscreen. The touchscreen does reduce the contrast a little and adds a little glare, especially in low lighting conditions, but many people (myself included) find that it doesn't adversely affect their prolonged reading at all. I really love the touchscreen functionality and am prepared to part with a little (not much) screen quality.
4) The grayscale level is important only if you want to view images or comics on your reader, I think. You will have no problem with text even on the lowest grayscale.
I like my sony prs-600 a lot, but be advised that it only supports the two english to english dictionaries it has already installed. If you are interested in any other dictionary, maybe it's not for you. Mobipocket supports any dictionary (custom made or commercial) but only a few devices still support it. I don't know about the dictionaries of the kindle or the nook.
About your pfs: why do you think they couldn't be reflowed? Any text based pdf can be reflowed I think, even if not optimally. If your pdfs are not text but images, I would advise to use an ocr software to turn them into text. Smaller file size and more flexibility.
And about the refresh rate: Two reasons it's a bit slow. The fists is e-ink limitations, nothing you can do about that, and all e-ink devices have it. The second is processor power. From the three readers I've had I find sony prs-600 is much faster, because it has less lag between the click of the button and the actual start of "page turning". I don't find the actual refresh of the e-ink screen such a waste of time, it really doesn't take much longer than turning a paper page. It's just that we get impatient with electronic devices, expecting everything to be done in a flash