as many said, you should pay atention to buy correct 349 rim and spokes.
this need to change rims is good because you can really change your brompton.
example: you can buy a doublewall rims.
example 2:you can change your rear hub to other sturmey archer(there is one with 2sprockets and 5 inner gears) witch is the greates for brompton.
example 3: you can reduce your wheels to 305 and use the schwalbe big apple tyres (same efective size diameter thqn 349 kojac in brompton)(nees to adress the brakes)