there are some good software programs for the MPC1000, on the mac i think its called Pad187 and the PC is called BlueBox, or something.... basically they let you create Programs on your PC you can import into the MPC. it saves you time from manually building everything inside the MPC, although if you are sampling breaks and building kits in the MPC you'll probably get fast with the just building kits inside the MPC itself.
i would recommend the internal memory upgrade (check the sticky in the MPC1000 thread), this will increase the sampling time available to you.
i don't know about paying a lot of money for DVD's on using the machine, there's plenty of youtube videos out there to look at first....
a 2Gb compact flash card is pretty necessary and much easier than finding the hard drive upgrade kit (last time i checked my local Guitar Center had stopped selling them). then again i already had one sitting around i used for my digital camera so it cost me nothing to go this route
the JJOS is definitely worth checking out!
beyond the MPC itself, if you already have a keyboard and drum machine you may want to look into using MIDI to connect all your devices and PC together. there's a website called Tweakheadz, google it for home studio setups, it gets into the gnitty gritty of all the components and ways to hook em up! a mixer would be a good investment if you plan to expand the amount of gear you have, but resist the temptation to keep adding new stuff before you learn the stuff you already own!