Hi
This link shows the work required to fit a Nexus 8 speed.
http://www.atob.org.uk/44/epage2.htm
The Schlumf drive seems to be very good but is expensive and may be difficult to repair.
The Roloff is even more expensive and difficult to install.
There is plenty of information on the Yahoo - Bromptontalk group.
The five speed mentioned would be worth investigating.
Edd
How hard is it to spread a Brompton's rear drop outs?