Brompton owners

I need some replacement spokes for the rear wheel of my folder. Are there any good sites which offer the ability to purchase individual spokes for 20" wheels?

Also, I fancy getting rid of my 6speed block and replacing with a single freewheel. Am I likely to encounter chainline problems?

Another one - It looks like it might be more cost-effective to just buy a new rear wheel, as the current rim looks to be near the end of it's life and spokes starting to show signs of rust. I measured my rear-dropouts at 130mm, but a lot of the 20" wheels for sale on eBay are designed for 135mm spacing. Is 135mm the standard for folders?

Edit - Didn't realise Brompton had their own spacing and use 16" wheels, so apologies this is off-topic!