Julian,

No, it is not particularly difficult to build for 144/432 MHz.
All you need are good low-noise devices, short leads and a little
determination, which I'm sure you have plenty of. For starters,
why not have a look at the simple 144 to 30MHz converter on this
site -


http://homepage.tinet.ie/~ei9gq/vhf_conv.html


This should give you an idea of the style of construction
needed. The only component that you may have trouble with is the
overtone crystal but there are other ways of achieving the same
end.


73 de Joe, G3LLV