Good choice! xsite do the OSW version, but it's fifty odd quid more expensive. Still, JD only have the carbine version in stock and if short is what you want then I reckon the folding stock is much cooler looking - it's just a shame that the battery doesn't go in the handguard b/c i hate wires coming out of a fake PEQ.
The only thing I would be concerned about is that with that gun you're locked into the CA mags for it - not that there's anything wrong with CA mags, the 5 G36 mags I have by CA are my best ones, it's just that you're not going to find them 2nd hand and if CA stop making them you'll be fcuked.
I'd be looking at running a minimum of 5 mid caps & 1 hicap as back up, which comes in at over a hundred quid on top of the mag you get with the three hundred & twenty odd quid gun, plus primary battery and spare, and a sling (too heavy to use without for long, but it would take a RS L1A1 sling for uber cool points). But a full loadout for big game days would need 8 mid caps & a hicap, eh?
In a similar vein, have you considered the G&G GF76's? Take standard STANAG mags I believe and the layout isn't quite the same as the AR-15 variants so it may well be more ergonomically pleasing to operate (if, like me, you find M4's fire selector and trigger placement awkward). Or there's also the G&G GK5C GL - 'kin sub zero!
Edit: ah, hang on, there's always something eh? In this case the 2 G&G's I just mentioned use V2 gearboxes and the CA SA58 uses V3, so that's preferable.
Edit 2: oh yeah, and I met the bloke who owns JD once at The Stan in Sheffield - he seemed to be a good bloke. He'd just helped a bunch of lads with a combined order for a few of the same pistols and BFG's. I have heard some neg-ish stuff since then mind, but I reckon that if you talk to the main man f2f you'd be fine.