Because there are so many options out there this is a hard question to answer without asking questions.
size:
Boards come in all different sizes from small 2 octave boards up to the full piano size 88 keys. 61 keys is a good starting point with unless your planning on playing piano pieces or have your keyboard split with different tones.
feel:
Different types of keys have a different feeling when playing them. Fully weighted keys are the piano style (mostly wood), semi-weighted are plastic keys with weights attached to the bottom and there are non-weighted keys (the least desirable).
Type of board:
Workstation, Synth, VA Synth etc..
Workstations are the work horse instruments which tend to provide you with all types of options such as sampling, sequencing etc.. Synth is somewhat of a generic term which for most people apply to the instrument below the Workstation, its a synthesizer which will have the ability to reproduce a number of various tones (piano, strings, synth pads etc..), Virtual Analog / Analog Synths are usually but not completely limited to making electronic keyboard sounds, and then there are stage pianos which are designed to re-produce piano tones as well as some other standard instrument tones. There are some other specialized instruments I'm leaving out controllers, organs, ep etc..
What you should figure out is a budget, and types of sounds you want then determine if you need sampling or sequencing or other options.
Personally from my opinion if you just want a simple board that you can dial up a bunch of good presets on to play Metal on, something like a Roland XP30 which I use and got one used for $400. There are similar good boards from Korg and Yamaha as well that you should be able to find in that relative price range. Of course the newer boards offer more options and will have updated wav sets (tones) but your talking twice the price for entry level instruments.
