Louie,
I would want to do the work myself as well. I'm sure some more experienced people will come along, but in the meantime here is something to think about. A relatively cheap and efficient way to take the willow trees down would be a brush clearing saw, something like the Stihl models they have on they're website. I have a Stihl FS90 string striker that can accept the brush saw attachment, and it works wonders on small diameter saplings, but it doesn't have the power to be cutting what your facing. I would look into their biggest brush clearing dedicated model. I'm not sure what the max cutting capacity is of the dedicated models however.
An expensive, but "oh so fun and fast" way to go, would be to purchase a PTO powered "Turbo Saw". This would enable you to very quickly cut the willow trees off at ground level, you could clear all those trees out in a very short period of time. However, at around $4,000 for the attachment, it doesn't make much sense if you don't have ongoing use for it.
Personally, I would think hard about a brush saw, maybe you could even rent one from a local dealership, these are pretty incredible tools as well and are quite capable.