Two years a go I restored a 70 Tiger. First bike I ever had and first "thing" I ever restored. With the help of some good, knowledgable people to do the hard stuff like honing cylinders, the build was fun and fairly easy.
All parts are available, and some things can be upgraded, ie belt drive, electronic ignition.
My biggest concern was the engine work. Hughie Hancox has a great video of him dismanteling and rebuilding a 1970 TR6 engine. Best thing I purchased by far. I had my laptop iwith the DVD in the garage and the engine went together easily.
Personally I don't know much about the other bikes except I've heard the conicle front brake for the 71 was not as good as the twin leading shoe brake on the 70. Having never ridden a 71 I can't confirm or deny this, it's just what people have said.
Another thing, 71 was an aweful color! Spring Gold is much nicer.
