Did you buy a 1500 dollar HD recorder not knowing what it does?
Eh, beside the point.
Okay, as was mentioned above, the inputs on the HD24 are line-level. If your synth has line-output, you should be able to run the L & R to two separate channels respectively.
For mics, you will need a preamp of some sort. One pre per channel of input you want. The M-Audio DMP-3 is good for the money (120-150 bucks I think mine was somewhere in there). You also listed no mics as far as I can tell... if you are running guitars and whatever into the machine Direct-Input, you'll need a DI box on top of the mic pre, but those are much cheaper - the Behringer DI (40 bucks) is actually a good unit, even tho the brand name would lead you to think otherwise.
As for monitoring - yes, you would need some sort of mixer that can handle as many mono channels as you plan to use (both record to and monitor afterward while adding more tracks) if you want to monitor what is going in, and I'm guessing you do. I have some ADAT tape decks that I use, and I run em out to a Yamaha MG32 - good board for the money (1100), especially if its just for monitoring. However, if it is JUST monitoring, you can probably get by with the cheapest 24-mono channel console you can find, which I think is the biggest Behringer Eurodesk they make, the 32-channel one (24 mono channels, 4 stereo). That thing's 550 I think... and if its just for monitoring, would probably do the job.
Hope that helps. Any more questions, feel free to ask!