If you spend more than £10 on HDMI cables you're an unadultarated mug. It's digital. It either works or it doesn't. Amazon Basics 2m HDMI cable for £3.99 does the job. Any of the digital cables - HDMI, DVI or Optical etc. are the same.
Wouldn't listen to what a salesman says about sound on a flat screen anyway - they're all crap. Spend £500 on a telly and £300 on an AV Receiver and some speakers for your budget.
Wouldn't touch 3D with yours either. Gimmicky until they go glasses less and produce more content for it. Unless you're going to watch Avatar once a day it's not worth it, and in 3-5 years time when they work out how to make Parallax Barrier work better, and can produce in larger sizes.