Is going to a meet an option for you? Major cities often host a couple every year. Look around on Head-Fi for announcements. It'll give you the opportunity to try a whole lot of different things. Just remember that show conditions can be noisy and difficult. Also, keep your bullshit detector on. I've had people at meets try to convince me of some staggeringly dumb things.
Also, seconding the advice to refresh your HD650: replace the earpads and headband pad, and it'll look and feel good as new.
The SR-007A is my favorite headphone, but that price you're quoting sounds too good to be true. If it's a grey market import, keep in mind that you'll have warranty service problems (which, granted, are unlikely to be needed; also, Stax has stingy warranties; anyway, asking for help here on HC will be more effective for some problems). The 252S amp is pretty good, but you might find yourself looking to upgrade to a KG amp, which will never be cheap, since they're all DIY (though not difficult to find). It makes for a pretty expensive system, but it can definitely be that "setup for 20 years" that you're looking for.
As a really nice alternative which doesn't require exotic amplification, the HD800 can be found for $800 or less on the used market in excellent condition. I've even seen it for around $1000 new, depending on holiday deals at various retailers. Add the (easy to install) SDR mod for another $30 or so. It's an excellent headphone, with much, much less bass roll-off than the HD650. It takes well to equalization, so you can give it just a touch to make it sound even better to your ears.
Dynamic amps are tough to recommend. I've never been a fan of Schiit amps for numerous reasons, but I don't have great or inexpensive suggestions that don't involve DIY. There's a lot of popular garbage on the market.
As for DACs, ask yourself: can you hear a difference between your current DAC and the audio output of an iPhone or iPad (with volume turned to the max) plugged into your amplifier? The answer should be enlightening.