Les, I don't think the extra parts are available yet for the sturm aircraft, the primary items being the armoured side panels, which you could make from plastic card, the side armoured windows, clear card and framing required also not hard, 30mm cannon brass tubing in the right diameter, shouldn't be hard to find. add cannon shell ejection chutes.At some time you would think that Hasegawa will add this stuff to do a sturm aircraft....maybe?
Dortenmann's aircraft is a good choice nice and colourful, as a companion you could also do one of his 'versions' of his personal D-9 W.Nr. 210003 which was the first Dora to be delivered, he used it for the remainder of the war until he blew it up at war's end, making it the longest serving D-9 as well. You have 2 colour schemes and 3 variations of marking during its service life. Markings are readily available in the kit or via Eagle Editions.
Some detail provided below