I agree with Charles and Hart in the comments that the size of the investment does matter, especially because you have no expected return on investment. Meaning, as you have no "followers" now, it is unlikely that buying a Tesla will result in automatic fame or notoriety. I would say that you cannot expense a vehicle to start a blog about that vehicle.
The federal government would appear to agree:
To be deductible, a business expense must be both ordinary and
necessary. An ordinary expense is one that is common and accepted in
your trade or business. A necessary expense is one that is helpful and
appropriate for your trade or business. An expense does not have to be
indispensable to be considered necessary.
It is important to separate business expenses from the following
-Expenses used to figure the cost of goods sold
I would argue that buying a vehicle is neither ordinary nor necessary. I happen to know a medium-sized auto review YouTube creator, and I know that it is normal to borrow or at the most rent cars. When they do a video on their own cars, they could expense fuel, but certainly not the cost of a new car. Further, the majority of the car's use will be for personal, and not driving to a and from clients or shoots.
Finally, although we've determined you wouldn't be able to expense a whole car for blogging about it, let's cover the rest of the question. First:
Assuming I am able to deduct it against this new side business, do I
deduct the down payment and monthly payments?
Normal accounting principles would say that you would deduct based on amortization. That is, taking the value of the car and spreading it over the useful life of the vehicle. In businesses where the vehicle is only used for company purposes, you would set a certain amount against your revenue. This decreases your bottom line, and would be classified as a tax shield.
What happens if my income every month from the site is $400 but the
car payment is $700?
Then your monthly profit is -$300 (assuming all else equal). Companies may reach negative profits by depreciation and amortization, and using them as a tax shield like I explained before is common.
Hope this helps!