First, be assured that while there are many enhancements on the Curve not available on the 8700, the 8700 is a great workhorse and STILL a very viable device. Granted, it does not have a camera, or an SD slot, Wi-Fi or GPS depending on which model, but it can be upgraded to OS 4.5 which makes the common features between it and the 8300 pretty much on par.
That said, the answer to your immediate question is it SHOULD be VERY easy to install Facebook and Live Messenger on a curve - DEPENDING....
Some carriers (most notably AT&T) still refuse to allow some applications. Now I've never heard of this complaint with Vodaphone, but the device you get will have a carrier code associated with it. If, for instance, you purchased an AT&T Curve, you MIGHT not be able to install AIM because it's restricted on the Download Page which MAY see your device as being from AT&T and not allow it to be installed.
I haven't heard of any restrictions on either Facebook or Live Messenger, and both are available to install OTA (Over The Air) by using your BlackBerry's Browser and going to:
Live Messenger: http://www.blackberry.com/devicesoftware/entry.do?code=wlm20
If you go to http://www.blackberry.com/instantmessaging/
using your BlackBerry's Browser and click the [Search]
button, all the "regular" (not FaceBook) IM clients available for your device will be listed with OTA links.
BTW - this should also work on your current 8700! Frankly, I think your "man at the shop" was more interested in selling you a new BlackBerry than giving you ALL your options. Believe me, if you have the money the Curve is worth while upgrade, but as I said before, there's plenty of life left in your "old" 8700. They wouldn't have made OS 4.5 available for it if it otherwise!
Hope that helps.