Hi Paxomen, welcome to MR!
There was alot of discussion about the best resolution to scan books over at
DIYBookScanner.org. IIRC the consensus was 300dpi color/grayscale was sufficient.
If you truly wish to future proof them, I would highly recommend performing some kind of OCR even though it does take more time to ensure a good final product. Who can say how long PDFs are going to be around and you would eventually have to OCR to get it into a usable/convertible format. Text with basic markup (ie html/epub) is easy to store, small file size, packages everything in one file, and is easily converted to just about any other format.
Cheers,