The HPI esc is the same as the Hobbywing V3, and does seem to be good but have heard a couple of people thinking that they're not as fast yet as the HW V2 or SP units, but this could be a setting or software issue. You can get either the HW V3 or HPI at good pries from Giant Shark, Demon Power Products etc.
Many people do seem to rub the SP GT pro esc, and it seems to be that it's a bit easier to install than most as the solder posts are low and can fit under the middle belt. This esc is about to be superseded be the new Reventon Pro so you should be able to get one cheap, JE Models seem to be the cheapest. They've been around for a couple of years but appear to be pretty bullet proof.
To be honest I wish I had gone with the SP esc, I have a Viper that has been a bit of a pain to install. So it depends a bit if you want the latest or happy to use stuff that is tried and proven.
The HPI motors are pretty damn quick, smooth with a nice top end. I have the 8.5 one in my Cat which can feel a little bit slow without any timing or boost. The 7.5 should be plenty quick enough.
The SP motors seem to be good as well, never ran one but many do. The fact that the motor wires are attached to the back of the motor with connectors should make maintenance easier as you can remove the motor easily without needing to get out the soldering iron.
I've never used Alturn servos but have had issues with Savox. The issue with the Cat is Schumacher don't use a servo saver so you either need a good strong servo with all metal gears or you can use a servo saver like this:
http://www.modelsport.co.uk/tamiya-h...products/29788
You could also try one of the Anderson servos that Schumacher distribute.