I guess this is a pool I can actually wade in. Like you, I also consider myself a novice. But I was told by everyone here “If you know which side of the soldering iron to hold; you can White Oak a Phase Linear”. I said “The shiny side right?” so the joke’s on them! But seriously; I would agree with those here who say go all the way! I know a lot of people tell you to forget the 400 and get a 700B, and honestly it’s like that old Monty Python bit SPAM. They all say you need to have that 700B but guess what? You may just not want it! Some people just want their trusty 400 brought to a new pedestal. I also have been told the 400 isn’t as harsh on your ears as a 700 can be. And it’s PLENTY of power. More than you’ll really need. If you can solder well, and know how to read directions thoroughly, then you can save a lot of money and do it yourself. That’s what I did. Just read, read, read a lot on the forum here of others who did their 400 upgrades. And explore the White Oak website. Then ask the right questions here during the build. You’ll find the guys here will be happy to help you. And I’d tell you $850+ and the pride you feel after doing the job yourself and the sound you get when it’s all done; you’ll never go back! Don’t let the naysaysers get ya down. I’ve read a lot of pooping on doing the full-monty on the 400 over the years here, and what it would cost you to buy the core 700, the upgrade parts for a 700, and then pay a guy to do it FOR you could put you well over $2k. I mean have you seen what fleabay going rate is for a core 700B these days? And that’s you paying for 45+ yr old parts you’re just gonna rip out anyway.
I say, you have the PL400 already so half the battle is won. Read a LOT, and consider doing the work yourself. You’ll find the documentation Joe provides is so good, you will feel confidant you can do the job right. I felt uneasy in the beginning too, but once you get in there and see what’s what, it all starts to make sense.
That’s my two pennies also. I hope you go for it!