What sorts of things might I need a technician for? "interactive" stages, but no documentation beyond a layout and schematic.
Well, if it doesn't work you'll likely need some help. Troubleshooting tube circuits requires some theory of operation knowledge. Voltage measuring, signal tracing, etc, and interpreting your measurements. This forum can/will provide a lot of help. Most problems on a new build are wiring errors, wrong component values, or sloppy workmanship. We've seen most of them.
What information or steps are necessary beyond following the schematic and layout?
You gotta have some chassis work skills unless you buy Weber's kit. Board work skills are nice to have also. You also need to know in which order to install certain things or which things to wire up first, especially important if you build this in a tweed style chassis. Having lots of pics of an actual build as it progresses from start to finish are nice to have too.
i.e. after following the layout and schematic, assuming good joints and no bone-headed mistakes (and/or finding and correcting any)... what tweaking, tuning, or modifications will I need and may not know how to do?
If you do it right there will be no need for any of this.
If you care about the quality of your workmanship, can read a schematic, and can follow a layout, there's no reason not to be successful. If you think you can put this together in one evening, or are sloppy, you will likely fail. Either way, this forum is a good place to share your project and you will find all the help you need.
Decide if you want to do it, choose a plan, and stick to it! Don't be jumping back and forth between Hoffman, Weber, and sluckey. Go for it! You can do it...