Which battleforce should I buy as a newbie player?
A lot of this comes down to how you build the armies. Many armies can do "long ranged hard hitting units with little focus on melee", but not all armies are good at it.

Most Space Marines (Space Wolves and Dark Angels) and Necron armies will be doing a lot at 24", since most of their army has that range. They can both be quite potent at this range. They are both not bad at melee, and can still get stuck in if need be (with Space Wolves being pretty good at melee).

I'm not totally up on the new Grey Knights codex, but I presume they will still be about the same as marines, and be more mid-range.

The Astra Militarum (Imperial Guard) are probably the longest range option, at least if you go mechanised. Between Leman Russ, Chimeras and Heavy Weapon teams (all three of which are in the Catachan battleforce I believe) you will be able to just sit back and fire away, with the footsloggers there to engage at mid range as the enemy close. Guard really do not want to get into melee unless you are designing the unit for it.

Of course, as mentioned, Tau and Eldar are better at long range then any of your options, so you may be in for some pain against that Tau player.