Airfix have gone into Administration, being taken down by Heller who were contracted to do their molding. They haven’t been bought out yet & their future is uncertain.
The Stuka, along with the Hurricane & the Mustang, are probably the best of the 1/24th scale Airfix Superkits. I have one about 60% complete, with another waiting for a rainy day & IMO, your initial inspection is spot on. The plastic is soft, but it goes together very nicely. Not bad for a mold that is around 30 years old.
Back in the 70’s, Airfix published a series of books to accompany their 1/24th scale kits to help build & superdetail them. Although long out of print (Airfix have already gone out of business once before in the 80’s), you often see them on ebay in the UK.
I don’t know of anything being released to detail the kit.
As for them “S” thingies, maybe have a look at the Xtradecal 1/32nd scale set, as it includes some in 1/24th scale. I got mine from Hannants in the UK, Here is a link to it at their site. http://www.hannants.co.uk/search/?FULL=X00232
As for Airfix’s future…Hopefully someone will buy them out. Some of their molds are tied up with Heller, but eventually, they should be released by Heller’s Adminstrators. Quite a few of Airfix’s molds are old & are past their best, and may never see the light of day again, but I would have thought that kits like the Stuka are the jewels in Airfix’s crown & would be released again. There is a case for many of the older molds being used again too though, as many of them remain the only option for some subjects in the scale.
Good luck with the Stuka & I hope that you enjoy it. I have got a soft spot for Airfix & their 1/24th scale kits & am glad that I stocked up on them before their current troubles. I’m also working on the Sea Harrier & the Mk1 Spitfire & have got the MkV Spitfire, the FW190, Harrier GR3, ME-109 & Mustang in the to be built pile.