DNG is the native format, lossless compression in raw Bayer whereas DPX is debayered already and more than 3x the size. The other reason you don’t want DPX is that it’s 16bit or 8bit, but the camera captures the information in 12bits (whether it gets all the dynamic range it can of course is doubtful) so to go to DPX you either have to pad the data to 16-bit or loose dynamic range to 8bit which is not what you want.
You can look at a less expensive lower-resolution option as well, you don’t have to go with the full 4K camera. The 3K one is also a good one.
Yeah I thought you might say something like that, but it will help if you set a goal. Haha. 
Using a projector as the transport is fine for a dailies scanner, but doing the old archival film you’re talking about will likely require multiple attempts to scan per reel very often, and that’s where the benefit will come in with using something that’s actually designed to scan film (if you can secure funding).