I know you've urged people to agree with your process decision but I'm with Ingrid on this..go shopping! I know how it feels to really really want to get started but down the track when you have to fiddle and sew and 'stuff' you may well think..arrggggh I should have gone shopping!
All that said, yes you could knit flat (or even in the round potentially) and add on later but unless you are felting etc the join may be noticeable. The best look may be seamless. Up to you really.