Hi Leo,
It just so happens that I am working on a small project that requires
a 16 bit DAC with a stable DC output.  A couple that I came across:
DAC714 made by Burr-Brown (www.burr-brown.com), MAX541/MAX542 made by
Maxim (www.maxim-ic.com).  All are serially programmed.  The BB part
seems to have better specs, but you can get free samples of the Maxim
parts from their web site (a definite plus, although you might run
into some taxes sending it out your way?).  As far as driving a
varactor, the BB part does 0-10V as is (the Maxim requires a
reference if I remember correctly? 5V 10V?) , if you need a greater
range use a good op-amp (see Maxim site :-) with a small gain to
mulitply the output of the DAC (use good (metal) resistors for
thermal stability).  Hope that is some help.
Bye
James.