Of all the classic serials that are still in existence, I think there are 29 that are still to be released on DVD so they would be easy to pick up. Some you can get fir dirt cheap, others you are looking to pay about £19.99 for. Some are more expensive than that though. There is also a boxset of miscellaneous episodes from partially lost serials.
If you want to watch online, there are places where you can find all of them, though the legality and legitimacy of them is questionable. BlinkBox are showing loads of classics for free and there is information about them from the news site
HEREDont think they have all available serials, but there is a good mix on there and stuff from all the Doctors, which will give you a taster and can help decide if you want to shell out on the DVDs (which you really should because the extras are awesome) and if you need any reccomendations there is a thread in this section somewhere where people have put theirs forward
