Reddit reviews Casio WK-500 76-Key Personal Keyboard with MP3/Audio Connection, 670 Tones and Power Supply

The Casio WK-500 is a decent starter for the price:
http://www.casio.com/products/Musical_Instruments/Workstation/WK-500/

About $250 street:
http://www.provantage.com/casio-wk500~7CASS1M5.htm
http://www.amazon.com/Casio-WK500-Personal-Keyboard-Connection/dp/B001FSJC2S

It seems like Casio WK500 would be something to get: it allows for creation of custom accompaniment, has a built-in sampler and a PA system (line - in), SD card slot and MIDI. I didn't get much time to play with it, but I am considering buying it for myself for quick drafts. It costs around $300.

As for the software - Energy XT 2.5 is a nice simple sequencer, and Ableton Live has an interface which explains where to click to do various things (but it might be an overkill).

In any case, you can get a free VST host and some free synths (my favourite being Superwave P8, MinimogueVA,Polyvoks station) - find all of them on KVR Audio Forums (in case you don't know, VST is a standard for virtual instruments, aka a way to get amazing sounds out of the computer).

You will also need a good audio interface for low latency (othewise the synths will be unplayable). You need one with ASIO support; use this google query to find one (Behringer ones are cheap and decent for a start).

http://www.amazon.com/Casio-WK-500-Personal-Keyboard-Connection/dp/B001FSJC2S

I have a used one of these...it's terribly improbable but if you happen to be in the houston area I would be happy to work with you. It has been sitting unused for a while as I haven't really been motivated to play it.

As far as finding one with weighted keys that's not too expensive will probably be tough as it's usually only the higher end ones that do. What is too expensive for you. Found what looks like a good one but it's about 400. Maybe watch for one on craigslist or something?

http://www.amazon.com/Casio-CDP-100-88-Note-Weighted-Digital/dp/B000GAP3J2