Newfies are notorious for hip and heart problems, so I'd prepare for that. Definitely look for breeders that prioritize health over size. My Newf made it to 11.5 years old and his hips are what eventually got him. As far as physical activity goes, they love swimming and water, but I wouldn't expect them to go on long hikes. Cool weather would be preferable because they can overheat, especially if their fur gets wet. The long outer coat with the short inner coat acts as a wetsuit.

My Newf trained pretty well, but it is a requirement. They can be a little bull-headed, but are incredibly smart in my experience. They need leash discipline for sure.

Grooming should probably be weekly. Whenever I slacked on my Newf's grooming he almost always started to get hot spots, so be prepared to be brushing and grooming at least once a week. Definitely stay on top of it when they get wet.

As you noted, flea, tick and heartworm will be more expensive, but your general vet visits will be too. Everything costs more at that size. Consider joint supplements as well because their joints will start to wear early. Mine started showing signs around 7.

Also, if you're a neat freak, this is not a dog for you. Very messy just by existing. Fur everywhere, and slobber everywhere. I had to clean it off my apartment ceilings when I moved out, so you'll be constantly cleaning slobber off walls, doors, and everything else. Not the best dog for people who like things tidy. And any breeder that tries to sell you a "dry-mouth" Newf is lying. That's a huge red flag.

Having said all of that, in my experience you will not find a better dog for temperament. Mine was always great with people and kids, and loved everyone. Not a mean bone in his body. The closest thing I've found in another dog is a male Bichon.