New Brompton 3 speed wheels can be found for £130 with careful searching.
You can usually sell Brompton parts.
You can possibly sell your hub for 35 pounds. And the rim for 10. Spokes 5.
Buy a good used wheel you might be in profit over spending time and effort and a possible wheel rebuild cost. Truing a wheel with 5 missing spokes is a waste of time. You could space out the missing spokes and then true it. Then replace later. But this seems a labourous task for little point. Cant you buy on line and just true it once?

But then what do I know about buying expensive folding bikes on a shoestring on ebay...