So long as you have a PC, MIDI keyboard, a prosumer soundcard and some nearfield monitors you shouldn't need the drum machine or synth.

Drum machines are something of an outdated concept nowadays; no-one really produces them any more (with the odd exception, such as the quite sexy machinedrum, or collector's items like the 909 and 808). VST plugin drum machines are another matter, and make a good complement to drum samples. Battery is a favourite, for instance.

Synths...if you produce on a PC, and you probably will given that your sequencer software will need to run on something, you'll have synths coming out your ears...it really is a bit of a non-issue unless you want a certain sound that can only be achieved on a specific piece of hardware (and that's pretty rare). I recommend shoving the idea of synths to the back of your mind...far more important is the sampler. In general, dance music production is about the sampler first and foremost - a sampler is the mocking bird of dance music production, and you can produce tracks entirely on it. Most folks new to production assume that synths and drum machines are the main concern, when really they're more of a supplement to the sampler (or bouncing to audio tracks for that matter). All the best sampler possibilities are on computers (with the odd exception, such as the Akai MPC series, although a PC + MIDI drum pad is probably preferable to that anyway...) but the range is surprisingly limited regardless. Kontakt, Halion, Reason's NN-19 and NN-XT, Logic's EXS24 and Ableton Live's Simpler seem to do most of the heavy lifting in this respect.

Of that lot, monitors will be the most expensive component, so do your most research into those. Soundcard, likewise do your research; check forums and the archives and reviews in online magazines such as www.soundonsound.com and www.futuremusic.co.uk ...you probably won't need DSP capabilities, just look for something with low latency and good user reviews with regard to lack of crackles and whatnot.

MIDI keyboards...again, do your research. Consider MIDI drum pads such as the MPD-16 and the Trigger Finger, and that combination drum pads + minikeys keyboard. A trick with MIDI keyboards is getting an old synth off of ebay. It's pots and faders may well be able to transmit MIDI CC messages, and the keyboard definitely will if it's not too old, and it may well be cheaper than a dedicated MIDI controller such as a Remote 25 and such.

Good luck...