The mall is a brilliant site. I played there many time when I was active. The crew who ran/run it were brilliant marshals and made the most of a unique site.
As for Sandpit, it was sold to me as "the only problem with the Sandpit is that it makes every other site seem rubbish".
If you get to the Sandpit I'll see you there (you won't see me though - I'm like a ninja).
Burrow your mates AEG and get a few games in. Once you've found your "home site", you'll have a better idea of what sort of kit you want to get.
Generally, were a nice bunch. Get chatting to the guys in the safezones, mostly were happy to show off kit. Handle a few guns (with permission, of course) and find what you like the feel of.
There is no "best gun", just what you prefer. There's more to the gun than you see on the websites, so it's a good idea to handle before you buy.
If you can get to Wolf Armouries in Camden, they're good for talking you through - or into - kit and guns.
Disclaimer: The ninja comment is totally unjustified and a complete falsification of the fact that I am, actually, useless at sneaking anywhere