A kit is a good way to go the first time. There is the Bottlehead and Mapletree of the top of my head.
The biggest stumbling block, however, is casing the darned things. Dilling holes for tube sockets, physical layout, terminal strips and binding posts, and a number of other essential little things are much more work than soldering the circuit together.
I'm not trying to talk you out of this, but I don't want you to buy all the parts and them stall because you're not sure how to do the socket holes or what to solder the resistors to.
Getting a kit is the best way to go at first. Once you see how it goes together, you can start working on your own designs and layouts.