If you have the time to do this.. go for it, and make a build log cause this sounds quite interesting lol.
I personally understand the fun of DIY. But I wouldn't wanna risk my components by creating a DIY watercooling system.
Especially since I only really have one main machine.
You should be a lot safer now that you're going for retail commercial made blocks though.
As for fittings, if you want to be cheap, get 1/2" ID barb fittings and squeeze some 7/16" ID tubing over it.
A 10ft spool of 7/16" ID tubing should be cheap at any hardware store. And at that point you can use zipties as hose clamps, but the tubing will be stuck on the barbs pretty tight already.
As for pumps, I've used an aquarium pump for a custom loop in the past, and im currently using a pretty good aquarium pump for well.. my aquarium.
The one I used to use:
(1200 model, $33, ANNOYINGLY LOUD, did not come with fittings from what I remember, I jammed tubing in the holes and used tape.)
The one I use now:
(L30 model, $38, comes with suction cup base that isolates vibrations very well, very quiet. also comes with 1/2" fittings)
Could probably find those pumps for even less at other retailers. Cant recommend any since you live in australia.
But yeaa have fun. Make sure your heater core is copper or else youre gonna have some nasty corrosion.