I would print it "as is".
My resin printer is an Elegoo Mars 2 pro, about three generations old in printer terms and my go to resin is Elegoo water washable transclucent - usually green but other colours are available.
For the S scale open wagon pictured below (GNR of england 4 plank) I went through a ton of prints trying to get it to print with a floor and in the end decided that for the sake of a bit of plasticard it was easier to do it floorless. As a bonus, it also sits horizontal to the build plate and reduces the print time.
Screen shot of supported model
a couple of views of one of my prints being turned into a work of art by Scott Willis
And the finished article
For your van in 4mm scale I would do the roof seperately, in 3 pieces, 2 end sections and the centre beam, not sure about the best way of doing the canvas roof doors. Axle boxes and springs easy enough to draw and print. Buffers print with steel heads (Gibson? Wizard Models?), W irons - use standard gauge etches but put a set in the W part, angle it out and then bend it staight down at the level of the bottom of the solebar (sorry that would be so much easier with a picture), it is only a mm each side. Brake gear, mixture of printed and commercially available etches.
I am looking forward to printing your van in S, Paul Greene will need a dozen or so for Broombridge.
Are you UK based? Are you coming to Railex in Aylesbury at the end of the month? If so please come to Trowland and we can have a chat.
Cheers
Rob