As an initial answer to your question about buying some credit and making it last forever. If you join a network on pay as you go and dont sign upto any of their bundles which offer you a certain number of mins and texts for 30 days, your credit should not run out until you have used it. I do remember a few years back 3 did set a time limit on how long you had to use your credit before you lost it, i will look into it some more to be 100% sure.