I frequenty use DAPs in my car. I would strongly advise against using any DAP with touchscreen controls (like the iBasso DX50/DX90) as a car player. A DAP controlled with a touch screen gives no tactile feedback and therefore requires that you look at the screen when you operate most of its functions. Trust me, you don't want to be fiddling with a touch screen and staring at the DAP screen when you're trying to drive. It's as dangerous as texting while driving, it not worse.
In my experience, the best DAP for use in the car is still the Sansa Clip+ running Rockbox. Note that the Clip+ has a button layout that is slightly better than the layout on the (otherwise very similar) Clip Zip. It's hard to explain, but the Clip+ buttons are just slightly better laid out, and just that much easier to manipulate with one hand without having to look at them. The only downside of the Clip+ is its tiny screen. You do have to take your eyes off the road for a few seconds to look at the screen if you're scrolling through folders to choose an album/song, and the Clip's tiny 1" screen is not ideal. I would suggest using one of the simpler Rockbox themes, and a thick bold font (Nimbus is great). The fact that the Clip+ only costs $30 doesn't hurt either.
Note that I'm not trying to take anything away from the DX50 here. I'm listening to mine right now with IEMs and it truly sounds wonderful once you install Rockbox on it, but I would never, ever use it in the car again. I tried it once when I first got it and let's just say I'm lucky to be alive.
I also have a FiiO X3. While the X3 does not have touchscreen controls, its button layout is very non-intuitive. It's not bad for use in the car, but the button controls take some getting used to. The major advantages of the X3 are its large display screen with big easy to read fonts. The X3 also has a line out, but be aware that its high 1.7V output can overload many car AUX inputs.
I also spent many years using an iPod Video (aka Classic) running Rockbox as a car player. The Classic doesn't make a bad car DAP, as the screen is a good size and easy to read. I just got fed up with the annoying LOD connectors which often broke, and the fact that it didn't have an SD card slot. Also, I realized that the control buttons on the Clip+ actually give better tactile feedback than the iPod's clickwheel.