CR2 is easier to drive and very predictable in its handling.
Most of the answers to your questions can be found in other topics here, have a look at them.
Not a single mod is 'needed', but you do need the XXX-NT bulkhead to attach the front end to the chassis.
Most people are using the idler gear mod, explained in the CR2 topic as well.
About the weight: in the UK they seem to use 15-30gr in the nose and a lead plate under the servo, I like a maximum amount of 10gr in the nose and around 110gr under the battery pack. I'm using a 20gr lead plate under the motor as well.
So what do you need besides the CR and CR2 kit:
- LOSA4160 Losi XXX-NT front bulkhead
- LOSA3079 Losi idler gear set
You just put a longer screw in the upper motor mounting hole, on which you put the idler. For the bearings used in it: take some old ones from a brushed/brushless motor, they fit fine. The idler mod will make it a 4-gear transmission, increasing rear end grip.
Set-ups can be found here:
http://atomic-carbon.gforceimages.net/cr2/index.html and
http://www.petitrc.com/setup/atomicc...CarbonCR2.html