It sounds like you want a Kurzweil K2600 coupled with a highly craftable analog synth, and a computer with softsynths. The Kurzweil is often overlooked because it's a little older than the other workstations. It's also the most realistic keyboard controller I've ever used. It pains me to admit it, because I love M-audio in general, but I don't like any of their weighted keyboards. The hammer action just feels loose to me, and the black keys sink down too far. Among synth keyboards, the K2600 feels the most like a real piano (outside of getting an actual acoustic fitted for MIDI, that is), which is what it sounds like you want.
And then there's the sounds. The K2600 has some of the best immediately usable bread n' butter sounds out there for a digital hardware synth, particularly when coupled with the right ROM upgrades (be SURE to pick up an orchestral ROM. It's jaw-dropping.) The sounds are also quite amazingly customizable for a digital synth, more so than the top three dogs (Fantom, Motif, Triton). IMO, if you want lower budget digital synths, the best options are the top desktop modules of yesteryear: Yamaha's MU series (MU1000 or MU2000 are best, but REALLY hard to find.
MU-128 is really nice too. MUs can use the same expansion boards as the Motif), Roland's Sound Canvas (SC-8850 or an
SC-D70. Avoid
the SC-8820 unless you don't mind controlling everything from a computer. Run like Hell from Edirol SD series unless you're just looking for "convenient MIDI"), Emu Proteus 2000 (some people like 'em. I like Yamahas and Rolands better).
Actually, about the best piano controller you can buy (dumb or otherwise) that feels like a piano is any Yamaha Clavinova. Since these come with very limited soundsets, they tend to be cheaper than a K2600 too, albeit not as cheap as a true dumb controller.
Now, for getting those crazy leads, pads, and scapes, look into getting a solid analog synth; something like a Nordlead or a Neuron. I would go with one of these if you can afford one in a heartbeat over any software analog-modelled synth (though "
Pentagon I" and Absynth can certainly be fun!). Keep in mind though, any analog synth is going to have a pretty steep learning curve before you finally start figuring out how to get the sounds you want. Better analog synths for less money are Korg's MS2000, Alesis Ion, or even a Microkorg (with vocoder baby!)
Then there's the softsynths. I have to admit being biased. For me, softsynths kill my songwriting enthusiasm, due to how much of a pain in the ass they are, between latency issues, loading times, CPU usage, and the fact that there's nothing to reach over and physically dial in unless you set up a bunch of external controllers for each one....Nontheless, facts are facts.
Fact: Softsynths are growing in popularity all the time, and are in some ways more convenient than hardware (realtime software plugins, and all)
Fact: For trying to imitate acoustics sounds as realistically as possible, nothing, and I mean NOTHING, is going to sound more realistic than a large digital software synth. MotifES, FantomX, Triton Extreme, K2600...none of the popular digital workstations come close to touching the realism of software like Quantum Leap, Vienna, or even Garritan.
Fact: I still sequence all my "realistic" stuff on my Sound Canvas first, and THEN render it with softsynths.
Obviously, "hire real musicians to play your brass parts and only use your synths for creative sounds" is the snobby answer, but let's face it: sometimes you need convincing realistic sounds, and it's not always possible to hire the real deal.
Hope that helps a bit.
EDIT: Seismetron - You beat me to it! Right on, man!