Hay back at ya!
What BoredGuy said plus a few more questions you have to ask yourself to find out what you need. The real-life difference between building a 5W 1X8" Champ clone and a 40W 4X10" is mostly size, weight and money. Of course there is a certain amount of added complexity in the circuit and more tubes, but not an order of magnitude. If you get into amps with tremolo and reverb, yeah, a lot more study is necessary to understand the workings.
You need to define 'gig-sized' a little better... in an auditorium with a loud drummer and bass, you'll need more than 25W. The 4X10" Bassman is physically big, and will put out quite a bit of sound. But even so, after a certain size venue you will either need a huge amp head with multiple stacks, or just mike it into a PA system, at which point a 12W 5E3 will be your monitor. So, you have to map out your needs, or you could end up with way too much amp, or too little.
The other consideration is your guitar and the kind of tone you're pursuing. Here on TDPRI, single-coil Telecasters and Tweed amps are the cat's meow!
Over at Strat-Talk you might get pointed toward a Blackface tone, and at MyL-P a British tone from a Marshall clone. I know that might sound a bit 'bigoted,' but it's relatively true. That said, some of those old Tweed circuits can be tweaked to make just about any guitar sing!
Here at Shock Bros., you'll find a lot of 'crossover' folks that know about most any imaginable amplifier, so whittle down your needs and these guys will help you make some decisions.
Finally, ANY amp you make with your own hands will make you proud! When BoredGuy said 'kit,' he didn't mean a snap-together model. Even a 5F1 Champ kit from a full-source supplier like Weber will challenge you. But Oh, the reward!
Edit: forgot to add, take a peek at this site Boot Hill Amps
. You'll see him here by the name Hackworth1, and offers great deals and lots of help to us TDPRI-ers.