I armed myself with a Dawes Discovery 401 cycle. It had the advantage of sprung front forks, which are useful, not just for off road, but Spanish roads themselves seem to have a lot of potholes, and hitting one going fast downhill is eased (a bit) by suspension in your front forks. Reading web sites by people who have made the trip without front fork suspension, the consensus seems to be that you should have it.
The 24 speed Shimano Gears with rapidfire Shifters seemed to me to be fine. At the start of the trip I used the low range, but by the end of the trip I was just using the high range. Certainly on another trip I would consider removing the small main cog, and replacing it with a larger one than the present large cog.
The bike came with Continental Country Ride tyres. Certainly I would not recommend anything lighter than these. Spanish roads are a bit grim for detritus, and I did suffer a series of punctures over a couple of days. Life being what it is, the punctures were all rear wheel punctures. This resulted in having to buy a new outer tyre in Plasencia, after experiencing 3 punctures in quick succession. Then a couple of days later having to replace that with yet another new tyre at Salamanca, after 3 more punctures.
Saddle. The bike came with a seat suspension post and a "comfort" saddle. All I can say is that I would not like to try a long distance ride with anything less than this. Certainly for the first 4 days I suffered from saddle soreness (and I was wearing padded cycling trousers). My guess is that this is something that is unavoidable for the amateur that has not hardened up the necessary parts of their body with rigourous training over months, and even then the pain of saddle soreness is something you have to go through regardless.
GPS enhanced Garmin Edge Computer was a really neat little gizmo that I would recommend to any long distance cyclist. As they say in their literature, you just clip it on to your bike and off you go. It records you distance travelled, speed in real time, time since you started, clock, altitude, total assent for the day (with an altitude profile over the day's trip). When you get back home, you can download the lot into your computer, and it displays your route on a map of Spain. I did muse on the fact that to give me my speed, it was done by satellites in space recording my position change over a few seconds, then the gizmo calculating and displaying my speed. Implant one of these chips into every human being and Big Brother would know exactly where you were at any time.
Via de la Plata to Santiago de Compostela