Having first hand experience with the valve gunk, I believe there is no chemical that can get it off once it is already there. That stuff is nasty. The shiny black parts are really goopy and the rest is rock hard. That shit needs to be scraped off.
Ive only had the VTA can on for a few thousand miles. The damage was already done, but the VTA can will probably stop it from coming back as bad. Some is still inevitable.
Now for me rating this job, I am no mechanic, I learned everything I know on this car by installing my own parts and parts on friends cars, but I would say I am mechanically inclined and can usually figure stuff out. I only broke one thing doing this, and that was the clip for the stupi MAP sensor...I
HATE the clips on these motors. With that being said, from 1 to 10 where 1 is adding washer fluid and 10 is building your motor with rods, pistons, turbo, valvetrain, etc. I would say this is a 6. Nothing is terribly difficult, but you need to pay close attention to what goes where. You need to have good management of bolts so you know where they all go afterwards. I have never tackled a job as large as this so organizing all the pieces I took off was a bit difficult.
I think if you have installed all the parts on your car with no trouble, people come to you to get things installed, and you feel confident you can do it, I would give it a shot. Worst case is if you cant get the manifold off, all you have to do to reverse what you did is plugging in sensor and putting your mods back on, lol.
Just read up on everything you can about taking off the manifold. Look at pictures and ask a lot of questions. That is what I did and it seemed to work out pretty well