Brompton recommends some aluminum parts (bars, cranks, brake calipers?) be replaced every ~5000 miles:

"...we recommend aluminium parts, particularly safety critical components, are replaced within 5000 miles - sooner for vigorous riders, and especially after an accident or significant impact."

AFAIK this recommendation is unique among bicycle manufacturers. Over on the Classic & Vintage forum you'll find cats riding with decades old aluminum parts. Anyway, if this gives you pause when considering a used Brompton, particularly one with an undocumented history, you can factor in component replacement costs into the sales price.