I don't have a Brompton, but on my small wheelers 16"- 20" I have double rims and one set of cheap single rims.

In general terms the double rims are stronger.

I've used BMX Alienation Ankle Biter rims for rebuilding 20" x 1 3/8 "wheels, very good rims if you can find them at the correct size and discounted. Got mine from customriders who sell bmx bikes and parts.

I expect you'll need new spokes too?

Beic