Probably for most builders a propane torch is adequate. You should be able to harden and temper springs, shape small metal parts, etc. If you get into building locks from castings or from scratch, you really can benefit from a welder and some sort of forge or heat treating oven. If you expect to caseharden parts you need an oven or forge that can keep a part at 1400 degrees for an hour or more. You can use kasenit with a torch but you will never achieve the same quality or depth of hardness as you can with an oven or forge.
There are different torches for propane that are useful. I suggest 2 types, the standard tip that comes with most Bernz-o-matic torches and has a spreader attachment, and a swirling flame tip. The swirling flame tip adds air to the mix and really boosts the flame temperature. I use it to harden springs and make springs. I even use it to melt small quantites of brass and silver for casting. The standard tip gives you a smaller, cooler, more precise flame for small jobs and soldering. Also buy Mapp gas and propane. The Mapp gas will produce a hotter flame for hardening and light forging.