What to consider when buying new wheels for an old bike?

As Dave says, if you're buying new wheels anyway you shouldn't have a problem - it's 2mm either side. If you want to do it really properly you could spread the dropouts to 130mm (Sheldon Brown has a page on this too).

Also, don't assume you need to replace the derailleurs or shifters. If the rear mech has enough overall range it should be ok and if the shifter is friction rather than indexed it should work too. If you do go for a thread on freewheel on your old hub, shimano make a 14-24 6-speed one that gives a nice range in 2-tooth jumps. I used one of these with an 8-speed chain on a Gazelle Champion Mondiale of about the same age as your bike.

do you want to replace the wheels completely? if so the easiest route is probably some nice looking new wheels (say halo's retro, http://halorims.co.uk/products-details.php?id=WHHARSF&type=all&c=X) with an 8-speed cassette. your dropouts will be 126mm but i've never had any trouble getting a 130mm wheel in an old steel frame.

I think the Maillard freewheel (possibly the Helicomatic, as per my 1984 Raleigh Record Sprint) isn't the same fitting as other freewheels so be careful what you buy if replacing it, and if so it would require a different tool to remove and fit. Modern cassette type is different again, they slide onto a splined hub instead of screwing onto a thread. It's a bit of a minefield. Sheldon's site is a good place to start, and retrobike.co.uk forum folk may also shed some light.

Hub width is a factor in wheel choice - frames of that vintage often have 126mm OLN*, modern road bikes 130mm, CX and MTB 135mm and track-only fixed wheels 120mm. Front wheel should be much more straightforward.

Fortunately, any 6/7/8 speed chain will work. I'd recommend SRAM PC850 or KMC Z82 for inexpensive but good quality chains. Count the number of links in your existing chain and use the same number in the new one.

Your Huret will be friction gears (not indexed) so you could change the number of sprockets on the back and the rear shifter should still work if it's in good order. New cable inners and outers are invariably a good idea, note that brake and gear cables on current bikes have differing diameters so are not interchangeable, I don't know if this was the case back then.

* OLN = over locknut width, the external width across the hub or between the rear dropouts on the frame.

If it's that old, you may well be dealing with a screw-on freewheel rather than a cassette. Saint Sheldon, as is to be expected, has a page about it: http://sheldonbrown.com/free-k7.html