Payload means the weight of your gear you are loading into caravan. In your case this is 165kg which will bring the caravan up to it's max loaded weight of 950kg.
So the empty weight of caravan should be 785kg. For an old caravan that means completely empty weight so your 165kg payload. includes gas bottles & battery. Also mover if one is fitted.
The 85% ratio would be calculated on the maximum payload weight of the caravan which in this instance would be 950 kgs. therefore you would need a towcar with a minimum kerbweight of around 1120 kgs.
just for your info. the weights you have just quoted, on a newer van would be shown as the MIRO (as from the factory) & MTPLM ( which is the maximum caravan loaded weight) the difference between the MIRO & The MTPLM is the allowed payload including gas bottles, battery, & any other fitted accessory not fitted as standard
Mass in service varies, A driver and fuel. 75kg driver and 3/4 tank of fuel as a guideline but it does vary between manufacturers and models.
If your putting 100kg into the van and you will do that easily you need to check the actual weight. It could weigh a lot more than the plate says already.
My handbook says it could be +5%, but someone wrote into a caravan mag saying their van was way heavier than that and left them virtually no margin for loading.