Taking out a wall-hung or pedestal basin and replacing it with a vanity unit is part plumbing and part carpentry.
You will need to disconnect the old basin, cut a hole in the surface of the vanity unit to take the new basin, fit the basin and reconnect the supply pipes. Before buying the ready-made unit, measure the available space carefully, then choose the counter-top basin to fit the new unit. The taps, basin and cupboard can all be bought separately, but to ensure that the complete unit is compatible it is obviously easier if you buy everything from the same source.
The counter-top basin fits into a hole in the top of the unit; usually the manufacturer supplies a template for cutting the hole. There are several different types of basins: the self-rimmed ones overlap the counter tops and are supported by them; the frame-rimmed model is secured with lugs that connect frame, basin and counter top. The unrimmed recessed basins are held by bolts and metal flanges. All must be sealed with mastic silicone sealant. You will also need a slotted waste connection, an overflow fitting, a suitable trap, and tap connectors to enable the final connection of the water supplies to be made to the taps.
Corrugated flexible copper pipes (15 mm) facilitate easy connection of the supplies, especially in awkward places, and are obtainable with tap connectors already attached. Integral ring-type fittings can be used for all joints. They cost slightly more than end-feed fittings but this factor is offset by the ease with which they can be installed: using a blow torch, you need only apply sufficient heat to melt the solder and the joint is complete.