I bought a collapsible 25' aluminum flagpole at the air show in Osh Kosh many years ago. I bought that style for the same reasons you list and I have been very happy with it.
I don't remember the brand name but I've seen the style many times at home or remodeling shows--you know the type of event.
If you are not 100% sure of underground utilities locations, call your electrical company for advice. Here in Missouri, we have 1-800-DIGRITE. They come out within 48 hours and mark the underground utilities at no cost. Hitting a TV cable underground would be inconvenient but hitting the 240 line from the pole to the house would make for an interesting afternoon.
Installation involved a hole, concrete, a 2' level, and a PVC sleeve. (I've learned to wait on the beer until after these types of high-tech projects are completed) The inside diameter of the sleeve is close to the outside diameter of the pole. We're not talking about .000002 clearance here but a sliding fit. I mooched an 8" 3-point post hole digger from my neighbor and bored about 3' deep or however deep you need to end up w/ the sleeve flush w/ the ground. Plumb the lightweight PVC sleeve in the hole and fill with concrete. Basically, it is the same procedure as installing a fence post. A bag of concrete mix from the local home supply, a little water, and a wheel barrow are all you need. If you like you could slide the sleeve onto the collapsed pole and then plumb the PVC. It might be easier to plumb that way.
Wait a couple of days for the concrete to set and slide the pole into the sleeve. Attach the flag and raise it on up. Stand back and admire your handy work!
If I had it to do over, I'd get the 20' pole instead of the 25'. It's windy here and the extra altitude seems to be tougher on the flags. I leave it at the 20' level now and the flags last longer--about 6 months for the cotton type.
Consider some way to light the flag, too. Mine is situated about 50' from the deck. I mounted an outdoor spotlight w/ a dusk to dawn switch on the deck facia that illuminates the flag at night. An alternative would be a solar powered light that mounts at the base of the pole. Another system would be a light at the base which is hardwired underground.
On the scale of 1-10 for difficulty, I would rate this project as a 2. It took longer for me to find the level and mooch the post hole digger than it did to install the sleeve.
