You can use iridium and they will last for 50,000 kms easily. Recommended one are NGK IFR6J-11 but difficult to find in market. Alternative is Denso IK-20 or NGK BKR6EIX-11.
NGK BKR6EIX-11 will cost you around 3800 rs for 4 plugs.
Regarding fuel filter replacement , if it is recommended. To change at 105,000 kms then get it change. It is not very costly .
Regarding engine oil, use synthetic or fully Synthetic,.
I also using Liqui Moly Hi-Tech synthoil 5W-40 with genuine oil and air filter and change three items ( oil, filters) at 8k interval.
Engine flush will help you in cleaning old engine oil residuals.