That all looks fine and once you grind the welds back, seal and paint no one will ever know. The only one I'm a little unsure about is this;
Looks like there is still some rust that's not been chopped out.
As for the bungs, I'd just keep them OEM. No point messing about welding them in. My car is nearly 28 years old and still has the original bungs and they are fine, anyone who's spunking thousands on these turds isn't going to be using them as daily runners through the winter and that is was has caused these issues, lots of salt, water and neglection. So long as everything is prep'd properly, sealed and painted properly they will be fine. If you are set on welding, just do it the same way that the small bung has been done.
For finishing the floor I'm going to be using zinc primer, fresh seam sealer, a base coat and then heavily lacquer it all. I've used Hammerite Smoothrite in the past and wouldn't let it near the E30, really don't like the stuff, POR15 Chassis Paint is a far more superior product if you wanted to use something along those lines.
I'm up for coming over but the 26/27th is a weekend too early for me as I'm working that Sunday and will no doubt be hungover on the Saturday.