30# is the right stuff for SWFF. Although the good stuff is expensive, it's an investment and you will have it for years and years. The loosely braided squid line does not pack as evenly after getting ripped off the reel and if you are going to join us on our conclaves this year it will be a factor
The cheapest way to get backing is to go to a full service flyshop like our sponsor's shops - they have bulk spools and machines that accept fly spools using rubber bumpers. These count yardage and apply even tension.
Most stores will give you a break on the backing if you buy something... a great deal for the consumer if you ask me.
Beyond # test, biggest consideration is color. Flouro green will bleed into white and clear lines, as will most colors except for white - which shows mildew and graying effect quickly. Yellow is a good medium.
I prefer to buy the screaming flouro and let it bleed into my line. Although it nulls the visual of line / backing joint I've always been able to tell where the knot is by diameter and the klunk-klunk-klunk as the albright flies thru the guides. I use the dye-infused back end of the running line as a warning track signal. Whatever you get you want something visual once the line in out there in the rip somewhere and all you see is the backing lifeline to cling to. Buy the best you can find, like Cortland.
I tried squid line once. Only once.
Make sure you tie a big arbor knot and fold the loop over itself before you put it on the spindle. In case you ever get to the bottom of the spool, the double or tripled arbor takes the strain off the knot itself, which is not very strong.