Short answer: SpaceX will autonomously dock a spacecraft to the ISS. This spacecraft will then use its thrusters to reduce the speed of the ISS, leading it to "crash" into the ocean, specifically into the spacecraft cemetery which is an uninhabited patch of the Pacific ocean. Parts of the ISS will burn up and vaporize during reentry, and other parts will splash down and sink to the bottom of the ocean.
Longer answer:
You should really read this paper. It answers basically all your questions. That said, as a short summary for your specific questions:
Will parts be saved and reused for a new station?
No. The ISS is very old. The oldest components were designed almost 50 years ago, and are rather outdated and reaching the end of their service life. Neither NASA or any commercial partners think it's worth the effort refurbishing or continuing to use what are, in technology terms, "ancient" modules just to save a bit on launch costs. It is deemed more economical and conducive to mission success to simply send new modules with modern technology up instead for future stations or space activities.
That said, it is possible that select smaller components will be detached from the ISS and safely returned to Earth for historical preservation purposes (museums).
What would it take to send it on a slow path toward the sun instead?
Basically impossible. Due to the way that orbital mechanics work, the Sun is the most difficult and expensive place to reach in the entire solar system. For example, sending something to Mars requires ballpark 15 km/s of delta-v while sending something to the Sun requires maybe 42 km/s of delta-v. Now, consider that all we've been able to put on Mars so far have been things like the 1-ton Perseverance rover. If we wanted to put the ISS into the Sun, we'd need a mission which is something like 1200 times as powerful as the already high-performance mission.
NASA did consider putting it up into a higher orbit for preservation purposes, however this is also not seen as good solution because:
- Doing so would be rather expensive and require a large amount of fuel
- The thrust required to do so is not a load the ISS is designed for
- If unmanned, the station would rapidly fail and become incapable of dodging debris
- The station would essentially become a debris bomb when it is eventually impacted by something high velocity.
One thing quite objectionable is the idea of 400 tons of mixed metals burning up in a controlled descent into the atmosphere. When defunct satellites burn in the atmosphere, they leave behind chemicals that could damage the ozone layer and affect how much light Earth absorbs.
You personally may find it objectionable, but it's really not a big deal due to how little 400 tons (or any satellite) is. The amount of ozone damage that a single burning up satellite does is probably less than someone in the boonies tossing out their old Freon-filled refrigerator without disposing of the CFCs properly, and while there is a theoretical environmental impact of putting the entire ISS from orbit into the ocean, it's probably less than the cumulative environmental impact caused by the people driving to work at Johnson Space Center in a day or two.
The environmental impact analysis suggests no significant long-term impacts.