Aye the sump plug (don't forget the new washer) cans be bought from Renault for like £2. Halfords didn't sell it for my car when I asked. Halfords do sell the 6ltr oil containers for the old oil, think they are around £4 - £6.
Halfords also sell the Champion Filters, or their own brand, again which cost £5 - £7.
Oil wise, you would do very well to go here,
www.opieoils.co.uk and buy from these guys. Use the online form and they will recommend a decent fully synthetic oil. Top of the list are Silkolene and Motul.
Remeber before you drain the oil to have warmed the car up slightly so that it runs out nice and quick.
Cheers.