It looks pretty bad but really it doesn't matter whether you have a tiny rust hole or almost nothing left, the rusty metal needs replacing either way.
From what I can see you're looking at new inner and outer sills, rear arches and the little bit between them.
But, you ought to check everywhere, and I mean everywhere, else before buying any panels. Front cross-member/radiator support, chassis rails, wheel wells, rear quarters etc. Basically anywhere where two panels join, and then the bits in-between..
https://bongospares.co.uk/shop/body-parts-panels
Apart from the rampant rust and the horrendous cooling system (on all but the 2 litre) they'll give you no more trouble than any other 25 year old grey import mini van. All the best.
1995 2wd diesel auto AFT 'Montague' rear conversion.