Water is a substantial addition to the overall load. Most pilgrims who contemplate riding to Santiago will possess a bicycle and providing it is a reasonable quality tourer or M.T.B., it will be adequate. Those without a bicycle or cycling experience need to do some research. If you can find one, perhaps the best option is to find a sound dealer, preferably one who handles maintenance as well, and seek his advice. The latter has a number of cycling pilgrims in its ranks who will be happy to help.
Cost is a consideration.
Both touring and M.T.B. cycles can be made to measure, with all fitments to your specification. Less weight means more money, but in the long term also means less effort. There is nearly always a weight penalty with an M.T.B. because it is designed not to break when subjected to great stress. In my own case I have a very modestly priced M.T.B. and recall one experience on the Camino which would have irretrievably smashed a touring bike. I simply rode on, both self and bike totally undamaged.