You want to be getting as many visas on the fly. Maybe.

It sort of depends on where you're going and what you're doing and how long you're going to be away for. The crux of the matter is that despite the best will in the world your grand plan for travel may not come to fruition. They way may be barred either literally or figuratively.

That is, your plan may have to change even as you're going along. This is to many part of the charm of overland travel and to many it must be anathema. The point is that you can't be sure you're going to do something until you get there so you probably want to avoid as many up front costs as you can.

There is some qualification that needs to be made to this. It may be financially more prudent to get some visas up front. In some cases it might only be possible to apply for some visas in your home country.

Most countries are pretty flexible however.

My favoured source is the HUBB.