I just noticed this thread, but our buddy
@syscokid has covered a lot of my thoughts on this kit already. Mojotone have a good reputation when it comes to amp kits, & this is a complete kit, so aside from tools & soldering equipment, everything needed to complete the build is included in the kit, nothing else to source or buy.
Mojotone use Heyboer transformers, so you know they're good quality USA made transformers, wound as the original items were.
Would I recommend this kit to someone looking to try their hand at building? In a word,,, yes. You get building experience & end up with an easily serviceable amp that will last you a lifetime.
Regarding the 10" Jensen speaker, the blackface Princeton Reverb's speaker baffle is fixed to the cabinet by screws (not so the silver face), so could easily be removed & a 1/4 moon shaped cutout added to the speaker cutout, facilitating the fitment of a 12" speaker. Might be worth checking with Mojotone regarding this "screw fixed" feature of the baffle/cabinet.
Ask Randall Smith how many Princeton Reverbs he did this to in making the first boogies. It was one of these that Carlos Santana plugged into & commented "man, this little thing really boogies" thus creating the name.
Cheers