I'm an advocate of old skools diesel engines, but I have to admit that a lot of the old diesels now will have done about a million miles and probably won't have been looked after very well. (and all the ones that have been looked after will have been sold quickly or will be kept by the owners untill they die, I'm one such owner

) I know good condition 306 1.9 Dturbo's are becoming very hard to find now. They have all been tweaked up (although it's not always a bad thing) but it's usually been done a bloke who has no idea what he's doing and has probably turned the turbo all the way up.
A second hand CR engine should make a good use buy now that most of the problem have been ironed out as such. Pegueot 2.0 8v hdi's are probably best avoided as they will need the bottom pulley replacing every 60,000 miles as they do break. If it's between that and a 1.9 XUD then I'd have a 1.9 but if they did put the 2.2 16v HDi in the 306 (they never did at the factory) then I would have had one of those over the 1.9.
Fiat 1.9 JTD's are supprisingly reliable and refined. My dad's van has one and it's never missed a beat, and he raggs the nut's off it every day.
I suppose the best advise we can give you is that you need to find one that's been owned by an enthusiast. They will have cared for it and lavished lots of love on it to keep it mint but in turn they might be a little more harder to get hold of as the owners will be asking a little more for them but in my opinion it's worth the spend.