Also make sure a 5E3 is the amp for you?
Good advice. I love the 5E3 circuit and sound of it, but it is not a highly versatile amp in its stock form - pretty much a one trick pony - but if you like that sound, it probably does that sound better than anything else (IMO).
When I scratch built mine, I made some modifications to increase its versatility, like.
multiple levels of switchable negative feedback for increased headroom and tighter bottom
voiced channel 1 stock but added a switchable coupling cap after the first preamp to give it a tighter bottom end
voiced channel 2 similar to a Marshall JTM45
added switch selectable solid state rectifier
and a few other little goodies to improve it, including a master volume control.
These made it a much more versatile amp. Not sure if they would indulge that kind of activity in one of those building classes.
If you go it on your own, you need to know what you're doing - but then you don't need the class in the first place.