LeadFoot, out of your choices, I would go with the Losi. It comes with Spectrum TX, the Associated with a 'digital' FM Tx.

The buggies are apples to oranges mate. Both are very good. Personally, I'm not a big fan of HPI (and no, I can't quantify that with experience, it's just my personal preferences)

What does everyone at the club/track near you run and what does the Hobby Shop near you carry the most spare parts of? It's nice to have a LHS or mates that have spare parts they can help you out with when you're in a pinch....

When it comes to RTR, understand that there will be a few things that will let go pretty quick, like servos.

As another idea, have a look at the Jammin (Hong Nor) X2 Buggy.

Another thing to keep in mind if you're going to be racing, some clubs/race tracks follow specific rules regarding engine size.

Good luck mate...and again, welcome to the addiction.