nRF51 standalone

[Punit]

Is it a good idea to use nRF51 standalone as an Application Processor and also as BLE connectivity chip? OR Is it a good practice to use a separate Application Processor?

I will mostly be using the Nordic NUS...Please guide..

[RK]

Standalone. There's a few cases in which there's so much processing or realtime requirements that you might need a separate app processor, but they're rare. If you're only using the NUS profile it's pretty unlikely you'll run up against any issues at all just using the nRF51 on its own.

[wlgrd]

Agreed. And if processing power is an issue (which in your case it will not be), one could switch to the nRF52 :)