Awesome, looking forward to watching this one!
My 2c for what they're worth...
Donor amp worked well for me. It gave me the chassis, transformers and tube sockets, plus I got the rectifier and power tubes thrown in for the price. I would say it let me get started quickly but stripping it took forever so I'm not so sure. That said, you can source all the parts separately so doing it from scratch is certainly an option.
My advice is first to decide what type of amp you want to build and then focus your research around that and ultimately select/design your schematic.
If you do get a donor amp then that may dictate some of your choices, particularly around the power section, but you still have some flexibility. For me the choice was between a plexi copy and a fender blackface copy... I opted for the latter because I wanted more clean tone and it works really well for me but I'm going to be adding a plexi channel soon.
Here are a few more links for you...
https://robrobinette.com/Amp_Stuff.htm - Fantastic resource, particularly for Fender amps
https://www.ax84.com/faq.html - Amp building forum with many projects and much advice
http://www.valvewizard.co.uk/ - Good technical info on tubes and amp design
https://www.youtube.com/channel/UCuR4hQTXkG_KxozLxwPzEjQ - Treasure trove of information and amp build/restoration videos. His dry style may not be for everyone but I can't get enough. He doesn't have proper play lists set up but scrape through his videos and you'll find his theory videos around 3 or 4 years ago. This is where I started.
https://www.trxe.com/ - Online electronics shop. They do bulk purchasing from Mouser so are a good place to get the harder to source items like transformers.
https://www.facebook.com/groups/savalve/ - Local amp builders group, not very active but enthusiastic and helpful. It's a good place to get advice and to look for second hand goods.
One last note. You'll want to get yourself a decent speaker if you don't have one already, it will absolutely bring your amp to life. If you have the ability to build a combo that's awesome, if you're just building the head then it may be easiest to buy a second hand cab.