I have an '86 Peugeot Orient Express which is not too different than your Schwinn. The bike pictured below is very similar to mine.
I have not used the bike as a commuter, but I have put around 1400 miles (2250 km) on it in the past few years. It's a great bike, very well-built, with gearing similar to yours. I use it as a townie, when riding with the kids, etc.
My only real complaint is that the handlebars are flat. For short rides, I can live with that, but for longer rides (>20 miles/32 km), I don't like it at all. I like moving my hands around on the bars as I ride. I've even considered putting some road bike handlebars on it, just so I could move my hands to different positions when riding.
The only things which have stopped me from upgrading significantly are 1) my Peugeot weighs around 35 lbs (about 16 kg); 2) it would be difficult to use the current brake levers and shifters on road bars; 3) in for a penny, in for a pound: if I am going to upgrade, why dick around? In other words, I would want to get rid of the heavy bolt-on wheelset, the drivetrain, everything. That means some serious dollars.
The frame on that Peugeot is Tange gaspipe, which rides great, but I think putting big dollars into upgrading would, in some sense, be like perfume on a pig. It borders on false economy, considering that a much nicer, lighter, more modern bicycle can be had for just about the same money.
The Meridas you linked look OK, and if you can afford decent components, that might be a good choice. Here in the USA, you can get some pretty decent deals on similar bicycles from online sellers. I don't know if there are equivalents in Australia.
But if I throw in my $0.02, assuming you will be riding the bike a lot-- commuters can really rack up the miles-- and that you can secure the bicycle adequately at work, I'd encourage you to get a full-on road bike. I know of riders who went with a hybrid bike and regretted it later, as evidenced by their road bike purchases, but I don't know of any who went with a road bike and later decided they would be better off with a hybrid.
If necessary, make do with what you have at the moment, and save for a better bike.