Hello there BertyG I noticed your post as you were wanting to buy a gaming PC. I will be buying one too in a few months time and it's also been a few years since I bothered with gaming - we will be spoilt for choice that's for sure!
My last PC was bought from Scan, the company you mention in your post. It sounds like you want to build your machine yourself (something I could never attempt). I unfortunately am no pro when it comes to PC parts and like you, also don't know much about certain bits inside my machine. I do recommend you perhaps base your build on one of the cheaper gaming PCs which they have pre built though, for example, the £630 3XS Custom Gamer 5i & then maybe customise any bits if you need too. Then try to buy the parts as oppossed to the pre built version.
GXF card. £220 for a 660ti seems excessive. Amazon £170, 50 quit to spend on something else. Shop around if you're truely budgeting :)
SSD. I have an 80gb, containing windows 7, photoshop and a few essential programs. MP3s, photography go on either my standard 500gb hard disk or my external 2tb passport drive. I'm not sure I would put games on an external drive...I just don't know about this? I could if I wanted, install a few games on my SSD and then uninstall them once finished. Maybe you could too?
Anyhow, hopefully someone else will pop by and give a bit more help, all the best with your build.