Hi,
13 days from Sarria means that you will an average of less than 10 kms a day. I'm in the process of helping some friends plan a camino from Sarria, and they too are walking slower than average but a little faster than you plan. We're using the mundicamino.com website for lists of accommodation. There are lots of listings and it's a good place for you to start. You can see the distances, elevation gains, towns in between, lists of albergues and pensions, etc.
The first day from Sarria, I can tell you, you have two nice private options. In Barbadelo, about 5 kms from Sarria (so much less than 3-4 hours) there is the Casa Barbadelo
http://www.barbadelo.com/ About 12 kms from Sarria, you will find this lovely place
http://www.casamorgade.com/ I'd suggest you take a look at the mundicamino maps and listings, because with distances as short as you would like you may need to find some creative alternatives.
Are you planning to stay in albergues as well as in private pensions, hotels, etc? If so, that opens up more options for you. For instance, between Portomarin and Palas there are several towns with no hotels/pensions but several albergues.
About cell phones -- I think the consensus is that movistar has better coverage than vodaphone. Wherever there's a vodaphone store, there's a movistar store within spitting distance. Either one is fine, though.
Money -- I can just tell you what everyone I know does. ATMs for cash withdrawals. They are everywhere and it's easy. Just be sure to have some extra cash stowed away for emergencies because you may hit a small town with no ATM or one broken ATM, etc. I know some people who only use ATMs when the facility they are connected to is open just in case the machine eats the card. I've never had a problem, though. Paying with credit card is apparently the best option for exchange rate, especially if you get a no-fee card like Cap One or Charles Schwab (at least that's what I've been told). I have been surprised at how many small grocery stores, etc, accept credit cards and I always go for that option. I would never exchange money at one of those exchange booths at the airport -- bad rates, high fees, and then you're stuck with huge amounts of cash. I tend to take out 200 euros and then replenish when I get down to 30-50, but many people do it differently.
I don't think you need to worry about making reservations at that time of year, unless there will be a problem with places closing for the season. I've never walked that late in the year, so I'll leave that call to others.
Buen camino! Laurie