You can get camcorders with AV-in (digital pass-thru) at various price points – so budget does come into it. In terms of the conversion function itself I would not expect much difference in quality across the models. But the camcorders themselves will vary in the quality of video they shoot.
The cheapest one I know with this feature is the Canon MV960 (around £250). It is a decent camcorder for the money, though some people report issues with motor noise effecting the audio.
If you look at the Sony lineup, you have to go up the range to the Sony HC96 (around £450) to get AV-in… but this will get you a camcorder with a larger 1/3” CCD and better lens than the Canon.
Panasonic has some nice 3 CCD camcorders (GS300, GS500)… neither has AV-in though.
Or… if money is no object.. get a HDV camcorder (Sony HC3) for around £750.. it doesn’t have AV-in, but then also get one of these
http://www.dv247.com/invt/23453 to do the AV-DV conversion. or if money is really no object one of these
http://www.dv247.com/invt/23454/