Thank you very much for your reply. I'm going to use NE612 because
it's quite popular and easy to use. Double balanced mixers are better,
but they need vary good impedance matching and high LO signal level. I
don't have much experience in building RF circuits, but I think the
IMD performance is not as important in product detector, as in the
first mixer.

My concept of the receiver stage is that: bandpass filters - variable
attenuators - first mixer AD831 - quarz filter XF9/B (a polish
replacement) with 9MHz IF - IF block based on variable gain
amplifiers LMH6505 and preamplifier LMH6624 - product detector.

I would use AD831 also as a product detector but this part is very
hard to find in Poland so I only managed to get one piece.

PS. I have read your webpage that helped me much in building my own
transceiver!