Well depending on your current products, the following are well worth having!
x2 buckets
Clay Bar
Compound and Polishes for your machine (and different pads), I like the Menzerna range for polishes
Sealant or Wax
Iron Out and Tar Remover
Drying Towels
Metal Polish
Tyre Shine
Detailing brushes for paint, wheels and interior
The majority of the Auto Finesse range is very good and reasonable on price