"Namibia is a desert country, totally unsuitable for a cycling holiday". Well if that is what Lonely Planet writes, it is almost for sure a great place for cycling. Seriously: Namibia is a fantastic country for cycle touring. You can travel for weeks on sandy dirt tracks without ever seeing any tarred road. Wild camping is mostly no problem. The price you have to pay for this, is that you will have to carry lots of supplies because distances between places to stock up on supplies and water are often very long. Very high temperatures can also be a problem.
Namibia has a fantastic road network. All roads are well marked even minor ones. Only mayor routes are paved all others are sandy dirt roads. Their quality varies from very good to rock-hard corrugation or soft sandy tracks. All in all they are mostly good and well maintained. There is very little traffic on the dirt roads - fortunately as it is very dusty.
Even in remote regions you will find surprisingly often farms where you can camp (guestfarm). But many oft these are a long way from the main route.
Wild camping is almost always no problem although especially in the south everything is mostly fenced in.
In bigger towns there are supermarkets just like in Europe. Good dried meat and good bakeries. But distances between those places are often very long and so you will have to carry loads of supplies.
Water often has to be carried over long distances because it is very dry. But there is almost everywhere a farm where you can ask for water, although they are often far from the main routes. Always check carefully how much water you need to carry and where your next 'refill station' is. I carried up to 20 liters.
Currency is the Namibian Dollar NAD which is 1:1 with the South African Rand. The later can be used everywhere to pay too. ATM's can be found in all bigger towns.