Help Choosing DAC

Hi, I have a Marantz 8b amp which I want to stream digital files from my computer. I plan to purchase some speakers (probably B&W's) used for 1500-3000 and would like to find a DAC that makes sense in this price range. Do some DAC units have the preamp built in? I am wondering if computer to DAC to amp is the right way to do this. Not familiar with DAC brands or features at all. Also what file type is best to build my library with. Thank you.