I was in a similar situation - a multitude of projects to do and a quandary between hiring the jobs out, renting a machine or buying one. Last fall I purchased a
B2710 w/ FEL ($15,400) and BL4690A Backhoe ($5,540) with 16" bucket (other implements are a 5' KK box blade & a 6' KK landscape rake). At the present I have about 60 hours on the Kubota with a good portion of that using the backhoe and I am well satisfied (these hours will quickly be going up as the Ohio weather turns warm and dry).
Backhoe projects completed include laying a 75' waterline, many feet of open ditching, installing a small pond and digging up a 2' diameter tree stump with extensive root system. Digging and ditching are a breeze and in no way compare to my previous back-breaking attempts with a shovel. The tree required a bit to work but was within the capabilities of the Kubota (the root mass was almost the size of the tractor itself).
Future backhoe projects include more ditching around the property, trenching for electric, gas and additional waterline, laying a network of drain tile, digging out a collapsed basement wall on my other house and waterproofing the basement/foundation of my home. Other than occasional ditch maintenance, the majority of my projects are a one time job.
My reasons for purchase were: I like doing for myself (labor can be expensive) and enjoy completing home work at my leisure; Rental would have been time constrictive and when the project was finished the toy (um, tool) would be gone; Comparing the price of a new tractor to used tractors told me that should I ever want to (not at all likely) I could sell the equipment and the depreciation would still be cheaper than rental or contracting out; As a firefighter I work a 24 hour on / 48 hour off schedule and can use the tractor and attachments to work for others on my days off and when I retire in seven years; I have two sons who are getting old enough to help 'dad' with some part-time work and make a little money for themselves; I have a wife who luv's me. ;-)
My initial impression about your situation is that renting a trencher for the power lines and contracting out or renting a backhoe for the tree & foundation work would be more cost effective than purchasing. I'm not sure if you could justify the $5,500+/- (or possible future sale/depreciation) of a backhoe for the tasks mentioned - but I do know you'd likely have fun using one. Take a good look around your property before making a decision though - you might find a few more reasons to have a backhoe within easy grasp.
As a sidenote: There has been a good bit written about the 3 point hitch backhoe's but I have no complaints about mine and only high praise....in fact, I am ~very~ impressed (and at times amazed) with it's capabilities and ease of operation. This comment can actually apply to the
B2710 / FEL and King Kutter implements as well.