Depreciation of any type, be it the bonus (Special Depreciation Allowance), SEC 179 or regular, has nothing to do with principle, interest, down payments, or anything of that nature. It has everything to do with $value$. Look, it's not like you learn this stuff through osmosis. yet it's also apparent you are attempting you educate yourself so you can make an informed decision. Smart move. But a public user-to-user forum such as this is not your best move. Many unwittingly attend a special school to learn it.... the school of hard knocks. I have multiple degrees from that one. 🙂
Also understand that any type of depreciation is not a permanent deduction. What may save you money now, can and will cost you more later. Sometimes as much as 3 times more. Google can be your friend. Or even better, you can find a tax professional with the heart of a teacher. You really need to educate yourself on this, but do it a better way...... not at the same school I attended. Way to costly and makes the cost of a professional educator seem like a pittance in comparison.
