Those are great suggestions.
One I would HIGHLY recommend (because I just bought one myself) is the Yamaha S03.
You won't find a single Synthesizer with as many voices for anywhere near the price range. In fact, most of the multi-thousand dollar Synths don't have as many either. The Synthesis of wave forms/envelopes, etc., is completely like most of the best line Synths, so You'll have total programming control.
Far and beyond, the sampled piano sounds (very many to choose from and play with programming and tweaking) are the best piano sounds I've ever heard, and I own several upright and grand pianos of concert quality and have played for over 20 years on supremely great pianos of all kinds.
The Synth is extremely easy to use too. It comes bundled with sequencing and other recording software and with waveform editors that let You do things normal, high priced Synths can't do. There literally is NO limit to the sounds You can produce. Yamaha has XG MIDI tones and programs that are top of the line, most far better than the sounds on the best samplers in the world and the best tone generators.
I absolutely LOVE my Kurzweil K2500, BUT, if I had known Yamaha was going to produce the Synth they have in the S03, I never would have spent over $3,000 for my Kurzweil. One of the only advantages the Kurzweil has is that I can play it live and have all my sequencing onboard along with all preset loops, programs, voices, etc. Whereas the Yamaha S03, I'd have to bring a computer or similar hard disk unit with me, or sequence the stuff and store the sounds some other way.
For recording purposes though, and for more direct, live playing, for a keyboardist who doesn't need multiple live programs and loops, the S03 is very very very hard to beat.
Also, it's the lowest priced fully programmable and professional Synth, with over 700 voices (each really great) already set up for You out there right now. I payed full price, $496, for a brand new one. Maybe You might find a better deal than that even, but the MOST You're gonna pay is just under $500.
Also, every last feature of the S03 is set up optimized for stereo output. This alone boosts every sound and function to professional and extremely rich sounding levels.
It's a 61 key board, but there's a set of Octave buttons that's very very well placed, easy to hit suddenly and always responds immediately, that shifts the entire board up two octaves completely or down two completely and all You do to return to centre is hit two of the Octave buttons at once. I'm completely addicted to it. I can split the keyboard up with several voices mapped to different sections of the keyboard and then hit the Octave buttons quickly and suddenly I'm using different instruments/voices on the board split. This is much faster than any other voice switching on any of my other Synths, except for pre-programmed voice switching...OR...if someone uses a foot pedal to switch programs. But You can do the same things on the S03, so, either way, it's incredibly fast while playing live or recording tracks live without having to stop the recorders and switch voices.
Of course, with MIDI mapping, You can do program changes instantaneously also, using the software that comes with the S03. The XG Works programs are almost identical to using other software like Cakewalk. We interchangably use our Cakewalk programs along with the Yamaha XG programs without any latency or problems whatsoever.
The ONLY thing that this Synth needs, that it doesn't have, is aftertouch on the keys. However, with the MIDI programming, You can cause aftertouch, with no delay at all. It's just not there on the actual controller in real time. Initial touch is though. And man, this Synth's initial touch is exactly my kind. Also, You can program the initial touch in every possible parameter. Yamaha even programmed in several general categories as references, like, with a couple touches of the Global settings buttons, You can set the keys to respond with more volume and accent if You touch the keys lightly and less volume and accent if You just barely tap them. You're completely in control of the sensitivity and volume curves, and so on. Besides this, You can pre-program sensitivity for 4 different filters in each voice. In other words, if You program/form Your own special piano sound, You could use one filter of the piano voice to have it so if You lightly touch the keys, that part of the voice will be soft, whereas another filter (part of that same overall voice) could have a violin in the background that bows its strings very slowly and quietly in response to the key touch, and at the same time have ANOTHER filter where a chorus of female singers saying "ooh, ahh, ohhhhhhh" sing, INSTEAD of the violin, if You tap the same keys harder. So, depending upon how hard You tap the keys, You could have a distorted lead guitar wail if You hit them hard or a nylon stringed guitar (and the sound for this is great on this Synth) pluck a string if You hit the keys softly....all the while, no matter how hard You hit the keys, the piano plays along with the other parts.
Again, the possibilities are literally endless.
I've met some people who don't like the feel of the keys, their action, how springy (or not enough) the keys are, but they're just right for me. Their action resembles an upright piano that is about 10 years old.
The sustain pedal (not included, but only costs about $18) opens up the sounds just like real instruments. My well trained ears cannot tell the difference. All this is before adding any effects like chorus or reverb. The preset modulations for each voice are pretty good already too, though You can alter them any way You like. The pitch and modulation wheels are ideal in response and springy behaviour.
I mean, for under $500, You get a complete workstation that You can do anything You like on.