You can still buy a B/K KT76A Mode A/C unit from HM for 1051 inc vat. You would be mad to do it with second hand units around for £2 – 300.
I have Mode C, and will put off mode s for as long as possible. The regulator has a three phase plan. Phase Three is ADS-B, and I may save big money by fitting a second hand mode s, or going straight for an integrated, approved ADS-B with GPS unit (which will probably also be mode s compatible).
Rod1