I am currently painting up various Orky vehicles although I don't have a real list in mind.

I am thinking of something along the lines of this for 4K :

Kult of Speed
Kult of Speed
Big Fortress mob +7 Boys and 1 Nob
'Uge Blitzbrigade (mix of Flak and Gun wagons)
'Uge Blitzbrigade (mix of Flak and Gun wagons)
'Uge Speed Freaks
Big Kult of Speed
Big Kult of Speed
Everything has a Mekboy Speedsta and the Kults of speed are 50% bikes, 30% Skorchas and 20% Buggys (ish).
Fighta Squadron
Fighta Squadron
Fighta Squadron
Fighta Squadron

I'm not sure how good the 'uge Blitzbrigades are. I used them in a 1K practice game and they weren't particularity impressive.

Is there any good source for Flakwagons? Ebay hasn't been very productive.

I find it better to have kult of speeds as mostly one type of unit. All bikes for assaulting, or all skorchas for firefights, or all buggies for plugging holes and plinking.

I have tried uge blitz brigades (but never two at once) I tend to use lots of small ones, but they are fragile. Maybe at 4K the uge ones will do better.

You haven't taken any outriders...these are a really nice unit in the speed freak list, as they have scout, a big unit of these can really get in your opponents way.