As Jon said, relationship with the store manager is a critical factor. We first got O2 from an "ignorant" Lincare store manager, but one who was willing to listen, read, and learn, and became exceptionally helpful. Call up. I worry that if you don't call, you might not get a tank at all, but a "concentrator" that makes O2 from room air. You don't want that. Also, make sure they're giving you a nonrebreather mask, not nasal cannula or a rebreather mask, and, as Jon and Denny suggest, at least one big tank for home use and one smaller tank for portability.
(We eventually went the welding O2 route and it generally has been better, except for the schlepping of tanks that Jon mentions.)
We provided the store manager (and the respiratory therapist who worked there) with several journal articles. The bedrock one is this study: https://www.ncbi.nlm.nih.gov/pubmed/19996400 While it calls for 12 lpm, most regulators that go up to 12 will also go up to 15, which is a good starting-place. (In my opinion, you will get some relief at 12, and maybe even full relief at 12/15.) On the right side of the page, under "Similar Articles," you'll see another article, about masks, and if you click on that one and look under "Similar Articles" you'll see several more potentially relevant ones. But as you say, just getting a basic system probably should be your immediate priority.
Another O2 mystery: Why would a doctor who knows enough to prescribe O2 without your having to ask, write you a 10lpm prescription? Amazing.