I've only ever really watched the stuff from 2005 onwards (though I've skimmed occasional clips on youtube), and I haven't felt lost, so I'm not sure where to help you there. The only time it might feel like you're missing is out is when they bring back/reintroduce classic enemies (Daleks, Davros, Cybermen, Sontarans, The Master, etc) and occasional allies (Sarah Jane/K-9) but a cursory glance at a wiki entry beforehand should be sufficient for that purpose (though beware spoilers for the new series, of course). Though the shows themselves usually do a good job of explaining who/what they are (a few friends I've gotten into the new series have only watched the new series material, and never anything beyond that and they don't feel lost). And it tends to be more referential to the characters of the new series than anything heavily tied to the old ones (Sarah Jane being an exception, what with her spin-off and all). So it's more about the people "you meet" (or "will meet") during the course of the new series rather than people you're assumed "to have met" previously. And, I suppose, knowing the concept of Regeneration in the context of the actors who've played The Doctor might help, but, again, it's explained in the series. Again, a quick wiki search for info on the first 8 Doctors can give you more information than you actually need to fully understand the new shows (though, again, beware spoilers for the 9th Doctor and beyond, obviously). I know they have put out various DVDs of TV movies/episodes or something (several with the Daleks, for example) of the older shows, though. Maybe skimming a few of those might be a decent prep before starting with Season 1 of the new series. But, again, I can't make any specific recommendations as I haven't watched any of them and very little of the new series has compelled me to do so (though I'll probably do so eventually at some point).