The FEVE doesn't sell tickets in advance, you buy them on the day. The journey from Bilbao to Oviedo is probably about 7 hours, the same as from Oviedo to Ferrol. We found about 4 hours at a time was enough (Santander to Oviedo) with a break in Ribadeo between Oviedo and Ferrol as it swings and shakes a lot. If you are over 60 you can buy a Tarjeta Azul for 3 euros which then gives you half price fares.
In Oviedo the Hostal Oviedo, Calle Uria 43 (effectively over the Cafe Cristina) is opposite the station if you come down the escalator not over the bridge. First solid door on your right on C Uria, with your back to the station clock.
In Ribadeo, if you decide to stop over, the Cafe Garden has rooms (also internet). Just follow the signs into town keeping right at the fork and it is opposite the big supermarket.
Between Betanzos and Meson do Vento there is nowhere to buy food or drink, although we were told of a bar in Leiro. Camping could be a problem as the woods are really forestry but others will maybe be able to advise better. Some of the tracks are very steep and narrow if you are carting things with your buggy and could be very difficult.
Apart from the long stretch between Betanzos and Bruma we had no trouble finding fairly cheap accomodation when not in the albergue. Also the Bar Zapatero in Leiro can give you the key to beds in the Colegio (4 only) according to 4 (male) Spanish pilgrims we met. (Johnny W - it would be good to know exactly where that bar is)
Hope this all helps your planning. We have had a marvellous time.
Buen Camino
Tia Valeria