I, for one, get your "logic" re: the GNC 255. The 255 is NOT a run-of-the-mill "basic" radio, it's state of the art with some very innovative features. More importantly it represents the next generation of non-GPS navcoms and makes a lot of sense to get into rather than last-gen stuff. You'll be able to build your entire panel around that radio and you'll probably be using it for another 15 years at minimum. I'd like to get one in my panel at some point.
You can get KX-155s w/ GS a bit cheaper than $2900, but not a lot. I agree, it just doesn't make sense to start so far behind tech-wise anymore. The price delta does not justify the decision.
It's a good foundation for when (not if) you add your IFR GPS. I've been strongly recommending people stay away from the 430Ws and 530Ws for awhile now, and going with non-WAAS makes no sense whatsoever. The bottom is just now starting to give way on the inflated prices held by the WAAS-upgraded GNS series. We're in the GTN era now. The cost of installing an IFR GPS from scratch is too great to not go that route anymore, all things being equal, in my opinion.
When you're talking about a very old piece of tech such as the GNS series, only the extreme popularity and "goodness" of the design have made it hang around as long as it has. That eventually fades. The market was also reluctant to move to touchscreen; think about the average age of the GA pilot/owner and it's not so hard to understand why this is. But the GTN series has been brilliant, people figure out the touchscreen quickly, and we're at the point now that one really doesn't hear about issues people have with touchscreen devices anymore. Our phones, our EFBs, even our computers are touchscreen now. When Garmin bet on touchscreen for the GTN series they did a good job of forecasting the future.
So once you have this upgrade done, down the road you'll want to pop in a GTN series Navigator and you're going to have an excellent base from which to build upon.
Looking forward to pics!