Should be more like 8 litres I would think for an M113.
Don't buy the oil from Halfords. They are too expensive. Buy Mobil 1 new life 0W-40 from Costco instead. They will be about £40 cheaper for the oil (2x 5L cans) at around £25+vat. Halfords charge £50 a can.
I would use a genuine filter - cheap from the main dealer and at least you have paperwork to show that it's been cared for with proper parts
Here's a link to the Mobil site which shows quantities for all makes and models, plus, of course, the 'Mobil' oil they recommend Which Oil for my Car? - Mobil? UK & Ireland
If you don't have a Costco store nearby, you could try eurocarparts (forum sponsors) and use the MBClub30 or MBClub25 discount codes at the online checkout, using their 'Reserve & Collect' option. The codes are subject to the value of the order and only work when they don't have any type of discount promotion on, as they suspend these when promotions are running. The MBClub30 worked when I bought 6 litres from them. Total £46.88
The eurocarparts Part Numbers for Mobil 1 New Life 0w-40 are (521771851 - 5 litre) and (521771852 - 1 litre).
As said, use a genuine MB filter. The one for my C Class cost £10.80 inclusive of VAT.
Click on the link in my previous post, it's either 8 or 8.5 litres, depending on the engine, but if your enter your Reg No., it will bring up the correct details.