Is there a good guide / video with instructions for replacing all the springs?

How about just a general orientation?  Ya'll use a lot of terms and acronyms that are new for me.

Source of gunsmith tools?

I am sure there are Youtube vids out there but I am not familiar with them.  Best BHP investment I ever made was to buy the late Stephen A Camp's book and laminated disassembly guide  I think his widow still sells them on that Hipowersandhandguns dot com site.

As for tools, nothing but a standard punch set works for me, nothing special required.

There are other authors, books, etc. on the BHP but Stephen Camp's work has been the most helpful, I also think a lot of his articles are still live at the hipowersandhandguns dot com site.

Pay attention to re-installing the recoil spring guide, you can get it wrong and it will mess things up.  There is a good diagram on Camp's site.

BH Spring Solutions has some excellent videos on YouTube and they make some handy tools for working on Hi Powers.

Also agree with Stephen Camp’s book.

This.

Thanks guys!

Careful with the trigger pivot pin. Only needs a punch, No Hammer, to push it sideways. Too many people beat up the pin, not knowing to move the spring out of the groove before removal.

I now have the Camp guide and his book as well.  Lot's of good info on his website too.  Thanks!

Now to get some tools...

If you have a MkIII or a late MkII, then you definitely want BHSS's specialized punches for the trigger pin.  They may not absolutely necessary (depends on your gun), but they will make your life much easier and you're less likely to ding up the frame.  Don't ask me how I know.  They would be my go to for anything spring or tool related.  They've got plenty of videos, too, and they're great to deal with.

https://bhspringsolutions.com/

All of this but make sure you buy that guide.