Being more steeped in having 2 different Bromptons around..
There are things Brompton offers like the forward turned saddle adaptor pin option ,(closer)
and fitting a lower Straight bar in an M bar steering stanchions..(lower)
and the Mk2 frames are a bit shorter from seat tube to headset, than the newer Mk3 &4,
the fillet brazed, instead of cast hinges were used in that era, & not so many colors..
Find red or with Black parts or all black [
afaik]
so, noting OP's budget consciousness, keep an eye out for a used sale..