I don't see how the "technology will be obsolete." A newer unit might have more interesting features, but GPSs made many years ago are still working fine. This is not like a PC where the latest and greatest apps and games take more and more compute power. And it's not as though they're going to change the signals from the satellites, suddenly rendering all existing receivers useless. No, a GPS you buy today will keep doing what it does, just as well as it does today, until it breaks.
One thing that WOULD become obsolete would be the maps stored in the GPS. New roads get built, every now and then old roads are closed, houses are built where there were none before, street names change, etc. If you plan to keep a GPS for more than two years, get one from a reputable company like Garmin or Tom-Tom, and be sure the model you buy has maps that can be updated. You will be able to update the maps in the future at far less cost than buying a new unit.
I think if you really need a GPS now, buy one now.
However do you really need one? Nobody has a built-in clue as to how to get around if they don't know the area. They learn the area by getting directions from people when they go to new places, and by looking at maps. Has anyone ever sat down with you and showed you how to read a map, how to relate what you see on the map to what you see around you, how to figure out from the map how to get from A to B? Or the basic layout of the major roads in your area? A GPS doesn't mean you can turn your brain off and just follow its instructions. Everyone should have basic map skills and it's really very easy to learn.
I travel a lot for work and these days I have a great GPS in my phone (HTC TyTN II). But I also always look up and print out general maps of the area and of the specific places I need to know (airport, hotel, client's site) and the routes in between. GPSs are great when they work but you don't learn anything from them - you don't "learn the area." You don't want to have to *depend* on a GPS all the time, and it seems to me that that's the direction you're going (pun intended).