i might be doing a century on it at the end of this month http://www.stcrispinsdaynightride.co.uk/
60 miles really isn’t that far though, some people tour the world on them (you can even get a Rohloff brompton kit!),
it’s probably cost around £3k so far (£1600 on the S2L-X and £1300 or so on parts and shipping)
upgrading Bromptons is quite expensive because they come with a lot of proprietary parts and while there are companies out there (like Brompification, Ti Parts Workshop and NOV Designs) that make quality aftermarket replacements for all the crappy plastic stock stuff, it’s still quite a niche market - so you’re not going to be able to pick things up second hand off ebay on the cheap. This means that you bascially have to pay full whack for everything and then more money on top proxying stuff from overseas where a lot of these companies are based.
I have seen some cheap ti and carbon fibre Brompton stuff on ebay but it’s all been J&L or Hylix stuff shippng from Taiwan - which I’d be a bit wary of personally…
in terms of a light brompton on a budget, the following would probably be pretty nippy:
single speed s-type with no mudguards or carriers, firm suspension and kojak tyres
then fit a ti or carbon seapost, brompfication pedals, lightweight Sella Italia or Fizik saddle and a light front wheel