What type of indoor shooting are you contemplating? Is this going to be paper punching? If so, then you are shooting at rimfire distances; there aren't any competitions indoors so far as I know for air rifles at that distance.
If you are sticking with indoors and you might be matched against 22LR rimfires then you would be advised to buy a 22 cal rifle. However if you are going to shoot FT or HFT 177 is de rigeur.
An S400 would see you able to compete at a good level in FT or HFT subject to some stock adjustments and enhancements. Your scope choice is more difficult, as the requirements for FT and HFT are poles apart, and it is not really workable to "compromise".