I would continue growing the lavender until they are as large as you can get them before transplanting, even if that means keeping them going inside through the winter. I can't remember if you are northern clime or south - what zone? Definitely build up the soil with as much as you can yet make sure it is well drained. It's easier to propagate lavender from cuttings than from seed. It grows into a low flowering, woody hedge, so be sure to give it enough room to mature.