This is what I'm looking to do in the (hopefully not-too-distant) future. The gauges you'd want (whether they're Defi, Apexi, or whatever) are ~52mm in size, and there are DIN gauge holders available for them from various sources.
Not too sure whether they'll fit in the gauge holder of your Stagea in place of the standard gauges, but I suppose if you can get the standard unit out and take it into a shop then you may have a better chance of finding that out.
The Defi DIN gauge you've seen on Nengun has three set gauges (you can't choose which gauges it comes with), and you can get more info here: http://www.defi-shop.com/product/din/din_top.html
There are cheaper Defi-style gauges around, with either the black face or a mirror face, but if you're choosing a boost gauge to add to your cluster, then they're mostly all mechanical gauges, not electronic ones. JustJap (http://www.justjap.com) and AutoBarn are two places that have the cheaper ones available, and JustJap also has a DIN-sized holder for them.
Hope that helps...
Nick