I made a bed-sized quilt for a raffle. I think it would be harder to use donated blocks because no two people sew exactly alike, plus somebody is likely to sew a really wonky block and then you have the problem of fixing it.
The quilt I made went together really fast once I had decided on the pattern and fabrics. (I guess because it wasn't made to satisfy my "creativity", I felt more free to just go with the flow, do the next thing that needed to be done, and not second-guess myself every step of the way. It helped having a short deadline too!) I first found a pattern I thought would go together pretty fast, and then pulled a few fabrics from my stash that seemed to work together. It was actually the fastest, easiest quilt I ever made! Looked great too.
My one suggestion is to make sure there is ample time to sell plenty of tickets. I think my quilt would have made twice as much money if more time had been given to sell tickets.