i am really torn on the 'best' way to do things like this.
1. i believe the only reason not to build your own pc is if you are really good at the software portion of it, but are horrible with hardware. honestly, i think the software side can be about 5x's more complex, so even that is not a very good reason..
i understand being new to the building computers-- i only started about 3 years ago, but most of it is pretty easy, and when you build your own stuff, it make it much easier to trouble shoot because you know where everything is, and while building, you can learn what every part does..
if you were to buy a prebuilt unit and something goes wrong, would you send it back for service every time?
2. because you specifically want to use the pc for datalogging and tuning, i would recommend looking up the specs for the programs that you use, and use that as a baseline for your requirements.
that package you were originally looking at uses a atom processor-- to some, it is sluggish, and doesn't handle multitasking very well. this could be a minus when dealing with real time data like with datalogging(i don't use my carpc for this so no idea on the actual usage)