Take the body off - obvious.
Ensure your transmitter and receiver have enough power.
Switch on Tx, then Rx, and see where the 'rest' position is for this set-up.
It should be with ~1-2mm slack in the rear brake, and the throttle rod as far back as it will go, without damage - obvious really as it will bend!
DO NOT START THE ENGINE!
Accelerate using the transmitter and see where the throttle moves to - there's ~90 degrees from rest / idle to full throttle in normal operation.
You say the lady who sold it to you had it idling fine, then what?
Sometimes, as in nitro engines, people might advance the throttle A LITTLE to aid cold starts, which isn't really necessary with large scale. Has this happened?
As I mentioned also, if you didn't get a user manual with your Tx, 'Google' the model and search for 'operator manual' 'operating instructions' 'user manual' ... you get the idea. You should find it without problem with some persistence. This will help you with the Tx / Rx setup you have.
'Digital electronic' systems as you call them are fine, you just need to know how to use them!
Good luck.
Alan.