Marine ply 30ft Yacht, Hull OK- Cabin Needs Repair

[hansp77]

After looking for months for my first yacht, I have just bought a 30ft (Herrod designed?) marine ply yacht (named Altair) with a Stuart Turner 8hp inboard and fixed keel.
I was not intending to get a wooden boat, for the Rot issues, but this one was just too good, at a great price, with absolutly everything (including 10 sails) and ready to sail. Someone had bought it for the mooring it was on, In sandringham, and wanted to quickley sell it to move in their power boat.
A bit of history (for those interested), the previous owner (before the guy who bought it for the mooring) was around 80 years old and has owned and raced it for around the last 40 years, regularly sailing it to Tasmania and back. This guy loved his boat, and treated it very well but had just got too old to keep on sailing.

Here come the questions..
The hull is absolutly fine, no rot, no soft spots, and has just been antifouled.
There are however a couple of softish spots (not too bad) on the roof of the cabin, and the biggest problem, a couple of leaks in some cracking seals and joints. There was about an inch of fresh water in the bottom, which I have drained out, and I want to fix the leaking roof ASAP.

This weekend, while I still have it on the jetty, I want to dig and grind out the seals and joints where it is leaking and fill/bog it up- then peel/scape and sand the whole deck and repaint. The question is, what with? I have had suggested polyfiller, or fiberglass. And the paint, Dulux weathershield?
The deck has been painted midnight blue, which could explain to some degree the cracking. I will repaint it white.

Also, to refinish the timber inside the cabin (not the hull) I have had suggested marine varnish.

If anyone has some valuable advice for me I would be most gratefull.

Thanks, Hans.