Pielit,
I'm not sure how that display works, or how you could incorporate in a case. I don't know if it alternates between displaying both voltages (input and output), or is user set to show only one. If you have to reach the switch to change the display, you'll have to consider that in your case design, along with the visibility of the display. Also know that most buck converters need a little overhead to work efficienly; I've been told to have at least 2 more volts going in than coming out. So if you plan to incorporate some type of internal battery UPS, it needs to be at least 7+ volts. The beauty of this project is its customizability. What you linked might be perfect for you. I personally would use a plain buck converter and add a voltage display similar to this
http://www.ebay.com/itm/like/390967087192?ul_noapp=true&ul_ref=http%253A%252F%252Frover.ebay.com%252Frover%252F1%252F711-117182-37290-0%252F2%253Fmtid%253D1588%2526kwid%253D1%2526crlp%253D53601919689_324272%2526itemid%253D390967087192%2526targetid%253D87946140729%2526rpc%253D0.18%2526rpc_upld_id%253D57680%2526rlsatarget%253Dkwd-87946140729%2526device%253Dt%2526mpre%253Dhttp%25253A%25252F%25252Fwww.ebay.com%25252Fulk%25252Fitm%25252Flike%25252F390967087192%25253Flpid%25253D82%252526chn%25253Dps%2526adtype%253Dpla%2526loc%253D9012452%2526poi%253D%2526campaignid%253D239125209%2526adgroupid%253D14978428809%2526gclid%253DCj0KEQiAjpGyBRDgrtLqzbHayb8BEiQANZauhxNM6J_yKKfuvO2d5t5J-LVP--ER1vwnJhQpvsHYwiYaAilh8P8HAQ%2526srcrot%253D711-117182-37290-0%2526rvr_id%253D928550818858&chn=ps&lpid=82
that can be incorporated anywhere in the case. They come in red, blue, green, yellow... you could have a green one for voltage in & a red one for voltage out. Its a little over the top for me for a Stratux, but I've done similar for other projects and it worked well.
Also know that a buck converter is at best 90% efficient, so you have to feed it more watts that you expect to get out. Shouldn't be an issue if your input is from the cigarette lighter plug, but a consideration for portable batteries.
If that link doesn't work, just to to ebay and Google "volts display".