I can speak quite a bit about the subject, my first amp kit was way back in 2009 from Metro Amps and was a JTM 45, since then I have done an Allen Accomplice, JMP 50 using the metro guide and parts from Valvestorm, a Trinity Amps 18 watt SIII and I just finished a Trinity Amps Tweed Deluxe and filmed the whole thing and am editing the videos right now.
Depending on your knowledge of how amps work is how you should look at buying a kit, I feel like the kits I have built were all very detailed and lead to a successful build. I know the mojotone and weber kits are a little less detailed, they are pretty much a schematic and layout which is still super helpful.
I would look at Trinity amps, I absolutely love what you get for the price and the service and instructions are outstanding. They have a number of good kits from Marshall style to vox to tweed fenders.
If you are looking blackface fenders then Allen Amps is the place to go.
I feel like sourcing your own parts for any beginner is a pain in the ass, that is why I love buying a kit. SAves me a ton of money paying shipping costs to 5 different places and you have a transformer designed for the amp you are looking for.