Hi,
This is a tricky question to answer, as it depends somewhat, on what kinds of sounds you want from your synth(s).
There are several factors which I think you should consider.
Make sure your intended synth can communicate fully with your software. I know you'll be using midi, but some midi software and synths don't properly communicate. I've only noticed this with Music-X II, but you should try to check that OctaMedSS aint going to have the same problems.
Consider the type of sound generation that you want to use. Some examples are analogue, virtual analogue, sample based, wavetable, fm etc. Read about these online.
Consider the number of voices you will be needing to play simultaneously. Some sound modules, or band-in-a-box type products can give you access to a large range of sounds, and are able to play a number of tunes at the same time (eg, entire percussion set, leads, basses, pads, etc). They may also offer effects, such as delays, reverbs, chorus, etc. These products can be great value for money, if you choose carefully, but generally don't give the same control over individual sounds, or quite the depth of character as a synth which is dedicated to playing only one patch at a time.
Take your time, and play the synth that you are considering buying. Demo it several times. By 'demo', I mean try to create the sounds/music that you want to use the synth for. Don't pay any heed to in-built demo routines in synths (you may see a demo button on a synth). These are generally crap, and not a fair indication of what an instrument is capable of. Don't believe the marketing hype either. Just use your own ear.
Carefully consider the level of control you need. Most people will need at least one keyboard to play. This could be from a synth with built in keyboard, or you could use a separate control keyboard, with sound modules, or rack mount synths, etc. One must also consider the midi implementation of what you are buying. Good synths will have just about every parameter editable via midi messages. As you are using old software, you should try to avoid synths that rely too heavily on system exclusive messages (sysex). It can be a pain if you need to send sysex just to change a patch or modulate a filter. Try to get something which offers knobs and sliders for the sound parameters that you want to modulate in real time. Some synths offer user assignable knobs/sliders, which can be handy. You can also buy/diy separate control surfaces, which are really great.
Is there a particular style of music you want to do? This can narrow the field considerably. For example, if you really only want to do hip-hop, using the most typical sounds, you could buy something like a JV-1080 (or it's recent replacements) as this has been a staple in the industry for so long (maybe not anymore, as I'm really not in the scene now). If you want to do acid tech. you would seriously miss something which sounds like a 303. If you want to do hymns, maybe an electric organ is for you. If on the other hand, you are highly experimental, then there is really no limit, other than your ear, your budget, and your imagination.
Budget, budget, budget... There are ways to get the most for your money, but it can be a real pain. Definitely patience is important here. It helps if you know someone in your area with a studio setup. A lot of musicians who have been running studios for some time can help you work out what would be good for you, on your budget. It helps to have used lots of different synths before, and have a good understanding of the different sound generation techniques. Often shops will not give you the best price, but second hand gear can also bite you. A lot of the classic analogue synths are getting unreliable now, and some also have issues with battery leakage, so be careful. If you buy from a shop, make sure they have a reasonable return policy.
Hmm, lots of other things to know. One good tip I could give is also to use PC's as sound generators. I have several old PC's which I use as software synths. I have dos boxes which I use just for a program called AXS. The computers were completely free, and the sounds can be really good. The only problem is the additional interfaces/cables, oh, and extra space too. PC's can be a great way to expand your studio on a budget though.
If you can mention what type of music you want to do, I may be able to give you a bit more help.
Good luck,
Oli