Go with a controller and softsynths. It will save you LOTS of money in the long run.

When you want new sounds you just buy new software and not have to pay for a module or keyboard every time.

Many of the developers are very good about upgrades, either for free or at a very reasonable cost.

You will not have to figure out the MIDI configuration of a new keyboard every time you upgrade.

You also won't end up with a wall of keyboards and a rack full of modules like I used to have, half of which were faves when I got them and then slowly migrated to the bottom of the rack as new faves replaced them. I never realized how roomy my control room actually was until I made the switch.

Good luck and have fun.

Well, it sort of depends on the sounds you want.

You've got 4 options.
1. Cheapo portable keyboard
2. Digital piano
2. MIDI controller
3. Synthesizer

Portable Keyboard:
The average portable keyboard (the ones with the built in speakers and rhythms) generally sound crappy. Some of them aren't too bad though. Because of the sound of these, I wouldn't recommend them, but try them out, there are ones that have a decent sound and you may find one that fits your needs.

Digital piano:
I consider a digital piano to be one of those keyboards that has a full 88 keys...sometimes weighted and is like a keyboard and stand all in one. They look like a little piano sort of. The sound on these can range from OK to really good. They will generally cover a wide variety of different piano and string sounds and some have other types of sounds as well. A lot include additional features like recording or accompaniment (sp?). If piano and strings is all you need and you can find one in your price range that sounds good, I would recommend getting it but make sure it has external outputs.

MIDI Controller:
A MIDI controller does not have any sounds. It's just a keyboard designed to control other MIDI devices (computer plug-ins, other MIDI gear). This would only be useful to you if you are recording into a computer and you had software plug-ins to use for sounds. You did mention that you could run through a PC before going into your recorder so this would give you more options than the portable keyboard because there is a large number of different types of VST instruments that you could use. However, if your just getting started with this and not really a keyboard player, I wouldn't recommend going this route...at least not at this stage.

Synthesizer:
Strictly from a sound point of view, these offer the most options. Synthesizers are (generally) professional keyboards. With one you get a great set of sounds, which can be modified, unlike on an portable keyboard or digital piano. A Synth doesn't require a computer or other MIDI device to use. You can just plug the outputs into your recording device. Synths include MIDI capabilities and as such can be used as a MIDI controller so they are great for future use. Some of them even include effects which can enhance the sound. For expandability and sound quality, I would recommend synths. But your price range may limit what synths you could get. You may want to look for a used synth or because your only looking for piano and string type of sounds, you may be able to find a new model that doesn't have all the bells and whistles that they usually have. Check out the Roland Juno D.