Stranded or solid makes no difference, even though stranded tends to be a little more flexible. If you plan on running it (or have to run it) by things such as phone line, power line, or fluorescent lighting you can get some STP (shielded twisted pair) which protects the line from outside noise. I used it in a run of 80' with a coax and phone line, had no problems with it (the cable itself is kind of stiffere than usual).
On purchasing, for example someone I know had the keen idea of going to CompUSA and purchasing 100' of patch cable with rj45 jacks on the end. He then cut them off and wired them up. That 100' cost him $40, which he could have picked up for 12 bucks. I go to
http://www.cableexpress.com for all my wiring, you can get bulk cable by the foot for as low as 12 cents a foot unshielded, and about 23 cents a foot shielded. They have plenum cable running from 32 cents a foot to 3.25 a foot.