TBH you're not going to get much new for 300 quid, probably not 7.2 and probably not Atmos.
You might get a budget Sony unit, 7.2 but NOT Atmos, maybe a budget Onkyo.
You might be better off looking for a used 2016 model instead.
As for Atmos, only really necessary if you're willing to invest in the in, on, or near ceiling speakers needed. The upward firing units that bounce sound off your ceiling can be very hit or miss.