I am a FAN of Horch trucks and flak conversions, and I will be following your build!
The DAK Horch flak-truck idea only festered in my mind (and that venerable Tamiya kit, and along with it, the first Tamiya Flak 38 kit, on my back closet shelf) for about 35 years (packed away 1973, pulled out to smell the glue in 2010)!
After seeing a few builds, I decided to finally do it - below is a pic of my truck completed June 2011... Sounds like your thoughts could have been mine oh, about a year back! How funny! Being an old-school guy and rather cheap, I did quit a bit of scratch work around the truck: tool mounts, lamp mounts, hull steps and details, lots of stuff in the driver's space, a whole windscreen assembly, a real wood gun-deck, some of the removable "celluloid" windows, and a new rolled rag-top. The gun is also the old Tamiya kit with an RB metal barrel, a scratched shell-basket, and various details around the gun and mount.
I found it to be one heck of a lot of fun! The old kit is really a gem, and there's lots of stuff one can do to make the girl sing. AFTER I completed my version, I found out more about the details one can add to that suspension, and I found I had missed correcting the rear doors and the driver's floor area actually needs more work than I gave it... (see some pics of museum specimens, etc.). There are plenty of things I missed.
Painting was with Floquil engine black (hull primer) and Weathered Black (rubber), followed by partial coat with lightened MM panzer grey (dunkelgrau = schwarzgrau) followed by a lightened Tamiya desert sand. I did some actual chipping and scraping to expose bits of the "dunkelgrau", weathered with some van dyke brown oil and some pin washes, and dusted it up with various pastel chalks. My decals had REALLY yellowed, so I stenciled the markings. It's been to a few shows and folks have apparently liked it. I may eventually post it as a feature or something here.
Aim HIGH! I look forward with great anticipation to what you do!