Don't wait, buy right away and get a frame built. You will end up spending way more than you think before you would be ready for multiday trips, so maybe for now you can focus on just getting a boat and the basic gear and do more day trips or non-permit trips.
To do multiway you will need an expensive cooler, toilet system, fire pan, tons of dry storage, first aid kits, the list goes on and on and on. I think I have way more invested in my accessories than my rubber.
Also, my advice is to buy good stuff once. When I started I bought some gear based on price, now I have re-bought tons of stuff so that I have the good stuff. Also, rocky mountain rafts might be cheaper and better than an NRS pro deal and they are plenty good. One more thing, I dont believe there are actually more rafts for sale in the off season than now, I always look at the classifieds and I think that there is more boats for sale now than there has been all winter.