I had a Royale mixte pass through my hands. It was a pretty great bike.

For any older bike, one thing worth a try is replacing the freewheel (gear cluster on the rear wheel) with a modern ramped one that is built for indexed shifting. Even if you keep your friction shifters, you'll find a freewheel with ramps will shift much easier than the old flat cog freewheels. If you want more modern wheels, you can likely fit 700C wheels without having to make major changes to the rest of the bike, as long as your brake calipers have enough reach (you have to move the back pads toward the hub about 4 mm, more or less). If you can find a 700C wheelset that has a ramped freewheel on it already... bonus.

If you need to carry stuff on the bike, there are several options. You can use a backpack or a messenger bag and just carry it yourself, or get a rack for the bike and get bags for the rack. You can get a trunk bag (a bag that sits on top of the rack) or, if you need more space than that, panniers, which are the bags that hang off the side of the rack, one on each side. You can get basic bags affordably at most bike shops; I got the basic house brand Sunlite panniers for my commuter bike and they've held up pretty well and are still in good shape after about 5 years.

You might want to read through this thread: Riding a 20-year-old Clunker to Work You might pick up some tips in there.