Ron,
Right now I have about 630 hours on my 110tlb and it is working well for me. The 110 is about the same as a 4000 series tractor's dimensions, but with the heavy built loader, backhoe, skidplate, heavy framing it weighs in about 7600 lbs. My unit with the Laurin cab is about 8,000 lbs. The 7200 lbs config is with a loader and boxblade.
The backhoe is easily removed but the loader is not, you can remove the loader bucket which has the skidsteer mounting. If fully equipped with all extra hydraulics, 7 function backhoe, top and tilt hitch this unit makes for a very versatile machine.
Whether to buy your own backhoe or not is a difficult choice to make, on the one hand the big jobs will likely need larger equipment, but the small jobs are almost impossible to do with a shovel. So do you hire someone else everytime you need to dig a small job? The other thing is in your area I would not recomend an ag loader, these green (or any other color) will not hold up to the abuse. The lightest I would consider is the 110tlb loader.
About the septic system, first of all I installed a large 3600 gallon system with 96 infiltrators near you at the Alamo School. This was a federal project and had full inspections along the way. Lots of rock there but you can peel it out in most cases with rock teeth. I dug this septic system and all the waterlines, electric lines with Deere 410G and a Case 580 with hammer. I also used a Cat 304 excavator to start with and it could barely dig a trench so it was swapped on the backhoe and hammer. Where we were working it was solid rick with a lttle windblown dust on top. Up near Pueblo Pintada north of there I have dug with a 3 ton excavator with no problems, so alot depends on your individual site. If your septic field is almost solid rock then it will require a large field to work. I can tell you that 96 infiltrators is a heck of a lot of them.
The dealer in Belen, I met him and that is where I saw my first 110tlb up close. He was hard to buy from though he wouldn't come off of list price at all and the dealer in Mesquite to the south below Las Cruces discounted my purchase about $11,000. I don't know about you but I can drive an extra 90 miles to save that kind of money.
When I bought my 4520 this year I first went to the Mesquite dealer and then the Roswell dealer. The Roswell dealer was about $2,800 cheaper than the Mesquite dealer, this was for the exact same (ordered) tractor. So right now the Roswell dealer has my business.
As far as the hammer is concerned I would call the rental companies to see what the day week and monthly rates are, that price seems awfully high to me. When I was renting from RSC I think a 410 backhoe (like new) rented for about $1450/month plus about $200 delivery fee. Doubt it would be much more than that now. The hammer rig was slightly more but an older unit, didn't have any problems with it either.
Hope this helps you some, I am a licensed General, electrical and mechanical contractor so let me know if you have any other questions.