A 'set' of points is a contract, not a Disney Vacation Club.
If you are looking at a 300 point contract with the intent of renting half of them out every year, you are looking at contracts that are too large.
You have no guarantee that you will always be able to rent points to non-owners, and you have no guarantee that you will always be able to rent all of your points every year.
You are better off buying a 150 point contract, or a 200 point contract - NOT a 300 point contract.
If 300 points fit your personal usage needs, then by all means - but 300 points. But if you are buying a DVC contract with the intent of profiting from it - then you are making a very bad decision. The only thing that DVC guarantees is the YOU the owner will be able to use your points each year at YOUR home resort. That's it - those two things are guaranteed - period.
You are not guaranteed the right to rent to a non-owner, and you are not guaranteed the ability bank or borrow, and you are not guaranteed the ability to book at resorts other than your home resort. Those are all things that you can do right now - but may not be able to in a year or two.
I understand your idea - but there are so many factors that are out of your control, it wouldn't be wise to buy more points than you plan to use, with the plan to rent half of them out every year.
If it wouldn't be a financial hardship to pay ALL of the dues each year, then you can afford to buy 300 points, BUT if it would crunch your budget to pay 100% of the dues out of pocket - then don't buy more than YOU will use.
I really think it is situations like this each year that will eventually cause Disney to tighten and/or create restrictions for ALL members.
Buy what you will use, where you will use it - that's the best rule of thumb!