It really depends how you plan to use the van and what kit your van already has. Will you be on sites, stealth, winter, cooking, need extra sleeping space….etc. I’d say don’t rush into buying things until you have tried it out several times and see how you get on. You can get by with stuff from home to start with (bedding, cookware etc). As you’ll see space is at a premium so you don’t want to be lugging stuff around that you don’t use. All too often you will see expensive awnings etc for sale that have been used once!
For me main things have been good are window coverings that keep the light out (we sleep in the van), the fridge to keep beer and food cool (plus electrics that allow it to run for sufficient time), an awning for extra sleeping space when there is more than 2 of us (and we are on a site), a canopy when only 2 of us, comfortable but compact chairs, a BBQ to cook outside, the diesel heater for winter use. We use the van all year round from stealth camping, pub stopovers to sites with EHU and what we take and what is essential changes depending on the trip/weather. Enjoy the van!