An easy way to check out some of the early Doctors is a special they did way back called The Five Doctors. It features four of the actors who played the Doctor and their various companions (including Sara Jane & Brigadier Lethbridge-Stewart) all taken out of time for one adventure. You can find it online for free. It was the first time I had watched any of the earlier Doctors and I found it pretty entertaining. It made me want to go check out the earlier stuff.