While it's possible, it's also highly unlikely that 26" wheels were ever on that bike. They are almost exclusively for mountain bikes.

Generally, as the quality of the bike improves, the wheels go from 27" to 700c. The difference is a whopping 4mm in the radius (a diameter of 630mm for 27" and 622mm for 700c, see Sheldon Brown's page, BTW, 700c is a size, not a measurement) . My guess is that the Batavus had 27" wheels, but the brakes may have been able to accept either wheel. The bike I linked to above appears to have 27" wheels (look at how high the shoes are in the brakes --- there's at least 4 mm for the shoes to come down to accept a 700c wheel). Do you remember if this was 12 or 10 speed? A 12 speed would have a rear OLD (Outer Locknut Dimension) of 126 mm. A 10 speed would have 120 mm. You can jam a 12 speed wheel into a 10 speed slot without ill effects, but it's best to get the right set to start with. You can put a ruler across the rear dropouts where the axle would go, and this would give the answer instantly.

You'll want to replace the cable housings and the cables for the bike. Modern housings have a plastic liner that reduces friction in the shifting system. Modern shifter cables have a thin plastic coating. Your cables are probably rusty in the housings, too. Shifter cable housing is relatively inexpensive, and cables (both brake and shifter) will run you $3 to $5 each. (NB, shifter housing and brake housing are different both in size and strength, so don't use one in place of the other.) Your local bike shop can sell you the right lengths of both in either black or white. For more exotic colors, you will probably need to go online.

For period correctness, I would go with the Normandy hubs on Weinmann rims. A 700c pair like this would probably work, too.

There are lots of decent online retailers for various parts: Nashbar, Tree Fort, and Niagara cycle come to mind.

And before you even ask, I'm going to recommend Panaracer Pasela Tourguard Tires with a kevlar bead. For 27" rims go with 1 1/4", for 700c get either 32mm or 35mm.

And whatever you do, just say, "No" to splash handlebar tape.