Also, there is a lot of good info on the original SCT410 thread that will address your questions. Because the .3 is an update of the original, a lot of the info concerning electronics will be applicable to both trucks.
But for a servo, I would look for something that as 200oz or more. Futuba, HiTec and ProTek are popular. You may need a short servo lead extension depending on what servo you use. My Spektrum servos need an extension, but I don't think Futaba need one. Also, get an alloy servo horn. The stock ones strip easily. The Tekno horns work great.
SMC batteries are very good an popular so stick with them if you already have success with them. I would look at the 7200mah capacity. I have never hit LVC in a 10min main with one of those.
There are a lot of good bodies out there. Anything listed to fit a Losi SCTE will fit this truck. Proline, JConcepts, Snows Design are popular.
You will find a wide variety of opinions on an ESC and motor. I run a RX8 and Pro4HD 4300. That seems to be a popular combo. But many run Hobbywing with a variety of motors. This truck is heavy enough to need 1/8 scale electronics.
If you are going to run 3s sometimes, I would get two motors. Ones that are good on 2s may not be great on 3s and vice versa.
Hope this helps get you started.
It is a fun truck, you will have a blast with it.