I fitted a brommie with a dahon stem extender and home made bullbars. The riding position was great like a road race bike and folded the same as a standard brommie. Have a look at the thread if it's off interest.

I agree with the comment try the bike out as you may hate the standard brompton ( or other bike) riding position.
Test rides recommended for compact folders always.
Sound like you may want to also consider a bike Friday or a birdie.