If you're booking TK business class, be cautious. You might end up on Indigo's newly leased Boeing 777. XD
Funnily, you can only book business class through Turkish Airlines even though the flight is operated by Indigo(codeshare).
Coming to the lounge, Diners Club International partners with Priority Pass for lounge access. So both your PP and DCB should grant you access to the same global network of lounges.