The one thing I would add is that many of the race oriented 1/8 scale Truggies are going to cost way more than an HPI 1/10 scale truck. I think the Xray would cost three or four times what a HPI Hellfire would cost.
If you do intend to actually race, I would spend some time at your local track and see what they run and which class might be most friendly to a new racer. Around my area, the Traxxas Slash and 1/8 Buggies are most popular.
Also I would try to pick the brand that is popular with local drivers. This way you know you can get help from them and/or find parts.