In my opinion, if your going to use it for more then a week, Buy it, don't rent it. Rentals add up so quicky its scarey. For a couple days rental, it could almost be 6months of payments if you were financing it.
What about just financing the attachments over 5 years or something similar?
I actually rented a Cat D5 dozer for a weeks worth of work, the bill ended up being $2400 from the rental company, now I need it again in the spring, so i decided to finance a used one to own, $2400 now covers 5 months of payments and I can use it more then a week.
About hydraulic contamination, usually its not a concern, all depends on the rental company and thier standards. I'v never had issues with that.
Jaxxson,