I'm considering the same but my new pond(s) (I raise fish for sale and need a couple more ponds) will only be a couple of steepsided pits about 50 feet by 80 feet. I don't see that terrribly involved.
The problem in my area with contractors is, out of five I contacted only three showed up to give me an estimate, and only one out of those three had a clue to what a core trench or keyway was. My last contractor won't be back as he didn't even scrape off the sod when he installed the dam on my last pond (I wasn't able to be around when he did that)and of course it seeps somewhat although not too bad over time. He also tried to pack it once the entire damn was installed which is pretty dumb.
If you do it yourself be absolutely sure you understand what a core trench is for your dam and do it RIGHT! From what my engineering texts say, a sheepsfoot roller is almost mandatory and the dam should be packed in 20 cm layers, and of course you need to dig a trench (core trench or keyway) under the dam which is filled back in, in layers. Nothing worse than seepage under a dam!
I have not actually contacted any big equipment rentals, but I would absolutely think it is imperative that they have some kind of insurance for breakdowns etc., and will come out and fix the equipment pronto.
Also get equipment that is big enough. Most of my local rentals don't have big enough equipement to do the job.
