A1. The KGSSHV uses some out-of-production transistors. While you can still buy them, fakes can be a problem, and reputable suppliers (like bdent) charge a premium. The Carbon is essentially a strict upgrade from the KGSSHV, with (I believe) all current production parts. The major advantage of the KGSSHV over the Carbon is the availability of versions with on-board heatsinks, which simplifies the metalworking considerably. You might consider building a KGST instead. It’s a great amp, and aside from matching 6S4A tubes, I found it generally pretty easy to make. Also consider the SRX Plus (though I haven’t made one of those yet). Search here for threads discussing these builds.
A2. Only you can decide that. It pays to try before you buy.
A3. The Carbon and Megatron are no more dangerous to build than any other device operating at comparable voltages.