As for the excavator questions, first, the 345B: does not burn 15 gal per hr, unless you are at near 100 percent duty cycle, which will not happen doing what you describe. I expect in your use, I would expect about 10 gal.
Second, the 345 will be able to do detail work. It is a fine machine for this type of work, with enough power for hard digging and rock, but good control. Parts and service are available, and will be for a long time.
Third, you can get most of the capacity you will need with a Cat 330, plus they are much cheaper to move and to work on. Look for one with a quick coupler, and you can pull pretty hard on a ripper than can be purchased for a reasonable price.
Fourth, steer clear of the off brand machines. While they may have ran good when new, there is little to no parts and service to keep one running into old age like there is with Cat.
I would not recommend any smaller than a 330 excavator for use in the rock, as they will struggle to get it done, and have more problems while doing it. There is a large population of 330 machines in use, so they are easier to find.
The Cat 330 matches up good with a D-8 dozer, and also down to a D-6. They are fairly economical to own and operate, and fuel burn at full load is about 9 gal/hr, with average work like you will likely do maybe 7 to 8 gal/hr.
Whatever you decide to go with, best of luck with it!