Hey Richard,
What a GREAT choice of build! A simple Tamiya kit but anything based on the 38(t) chassis is great imo which is why I have a soft-spot for Marder III's!
I recently built this exact kit myself and painted it in the Ambush Scheme but I over weathered and used rattlecan gloss varnish (solvent based) which Karl reckons really darkened my lighter areas of Dunkelgelb
Because of the Side skirts hiding the return rollers making visible sag pointless I was able to easily get away with using the vinyl kit tracks - they look ok. I did use an AM barrel but it wasn't really necessary as the kit barrel is one piece i.e. no nasty seam
Because of the darkened paintwork I've decided to do a full hairspray whitewash on mine. I'm yet to do a proper whitewash so it's a perfect base for it.
As for your build I'd recommend chipping and damaging slightly the edges of the rubber on them big road wheels. The damage looks really good if done properly.
I'd also look at your side skirts. Obviously Tamiya supply them as one solid moulded piece but I wish I'd cut mine into sections to show individual panels which they were like or even used brass replacements which are readily available.
What paint scheme are you planning on?
Bill recently built the Dragon Hetzer and painted it using straight Dunkelgelb and it looked SOOOPERB! He fully blogged his great weathering work so checking it out on his site might be of use to you and give you some suggestions?
Best of lucK - keep us posted!
ATVB
Ben