Thanks for all your suggestions.
I already own a Graupner MC-22, which is somehow a very nice radio (and it cost me around $700). I would like to be able to continue using it with both FM and 2.4GHz RXs.
My feeling is that I will move on to the Jeti Duplex JETIBOX PROFI system which looks very promising to say the least. It features an internal Rx/Tx, so no need for any extra module (that you have to install inside your Tx). You just have to find the 3 coorect pins inside your Tx, route them to the BOX and you're done.
Jeti is about to be sold in the USA I think. See :
http://www.espritmodel.com/jeti-cz.aspx
There is no LHS here, only online stores for me.