Originally Posted by berlinonaut

For the innocent eye nothing really changed. At a closer look almost every component has been changed over time at least once and details and parts have been enhanced massively over time. In any given year smaller enhancements have been made, every ~4 years bigger changes were made. Almost everything is retrofittable, but part costs sum up and some things are expensive. So in general a relatively young bike is the best deal, depending a bit from your budget and needs. Retrofitting a cheaper, older bike to today's standards comes out more expensive than buying a newer one (and sometimes more expensive than buying a new one). Prices for used bikes tend to fall more intensively in the first couple of years and at some point do not fall any further relevantly.

A very rough overview from memory about the more obvious changes (only a fraction of the overall changes):

2000 MK3 -> higher quality parts all over the bike in comparison to the MK2, new folding hinge on the stem, dual pivot brake on the front
2001 SRAM hubs instead of Sturmey, no more 5-Speed
2002 6-speed invented (with 2-speed derailleur)
2004 MK4 with longer wheelbase and new folding hinge
2005 S- and P-models invented, ti-models invented, SON dynamo invented, 2-speed invented. Sturmey SRF3 replaces SRAM 3 speed
2007 dual pivot brake on the rear
2008 rear frame clip as a standard, jagwire cables
2009 6-speed BRW replaces 6-speed SRAM, new saddle, pentaclip as a standard, right aluminum pedal, change of colors from gloss to mate
2010 Shimano hub dynamo replaces bottle dynamo option, aluminum seat post instead of ti one
2012 H-model, LED light on the front, no more aluminium seat post, only steel
2013 new brake levers, new rims, new spider crankset
2015 start of the black edition
2016 new rear carrier, new roller wheels, start of the nickel edition and various special editions
2017 new stems for M and H along with new bars and new under-bar-shifters
2018 new brakes, Brompton electric
2019 new hub dynamo (SP SV8 replaces Shimano and SON)

Along with those go yearly changes in available colors, a lot of smaller technical changes and a lot of changes in accessories like bags etc..

If you can afford it I would go for a 2013 or newer bike. If you can't and want a six speed I'd go for no older than 2009 (BWR). I would not buy a MK2 because of the old frame and outdated, lower quality components. I would not buy a MK3 because of the old frame, at least nor as a daily driver and if it was are not massively cheaper than the usual offers for Bromtons of that age. If you know what you are doing you could buy any MK4 but you should have in mind that - depending from the usage before - a lot of maintenance may occur like rims, rear hinge, chain + sprocket, tires, cables, etc.. This, together with desirable upgrades like i.e. the 2013 brake levers or the pentaclip, may quickly rush you into a price region that you can buy a newer bike instead. Thus I personally would not buy older than late 2008 with the exception of a ti or a cheap offer that is technically in above average condition. In this case calculate a reasonable amount for mainainance and upgrades (~150-200$) and rethink if it's still worth it.