Hi, jsalt,
I have walked from Lisbon to Porto, but it was a little later in the year. It is solitary and quiet no matter when you go -- I have heard from people who walked in mid-summer that they did not encounter any other pilgrims. The British Confraternity of St. James has published online guides of the Caminho Portugues, one from Lisbon to Porto, and one from Porto to Santiago. You can download them here:
http://www.csj.org.uk/guides.htm
(I am the author of the Lison-Porto segment, and as you will see, there is a brief update available as well, based on comments I've received from others who use the guide.)
There are many posts here on the forum that will give you some information, too. Take a look at all the listings under the Camino Portugues heading, there are several hundred, and I'm sure you will find lots of info there as well.
But the short answer to your questions, at least with regard to Lisbon to Porto is that accommodations are always available, but there are no albergues south of Porto. Private lodging usually runs about 20-25 euros per person, but there are places for pilgrims to sleep for a donation in the fire stations. I go to Lisbon for two weeks every year, usually in March, and the weather is tremendously varied -- sometimes lots of rain, sometimes lots of sun, sometimes warm, sometimes cold. I would be prepared for rain and chilly. And the way is well signposted except for the couple of places I note in my guide.
Buen camino, Laurie