Like Purehavoc said, if you see a great PSA sale on something you're going to need later, then you'd better jump on it now. I've missed out on their deals and really kicked myself later when I needed it. On the other hand, I've never been sorry I jumped on a deal if I knew I could use the item later.
As far as stripped lowers, it's a good idea to call around locally. You never know what someone may have in stock. You avoid the transfer fee and can buy it now.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek