Off to a good start, but could improve on the prep work.
Remove the mold lines off the auxiliary weapons, smoke launchers, tops of the drive sprockets/idlers, sides of the extra fuel drums.. Drill out the barrels (some look like they were drilled, but are way off center. Fill with putty and re-drill in center of the barrel. Use the tip of a hobby knife to twist in a pilot hole in center of barrel for the drill bit.
Is a Heavy Stubber (the cupola pintle mounted weapon) an option for Space Marines? If not, replace with Heavy Bolter/Plasma Cannon/Multi-Melta/Heavy Flamer. Look at the picture of the Main Gun. Looks like some spots that the sprue was twisted/broken off instead of cut and cleaned with a hobby knife/sprue clippers. Sand/file the areas, fill with putty (liquid greenstuff or Squadron green/white putty). Also double check the seams and smooth out by light sanding/file or scraping with back side of hobby knife.