Heres a topic that really helped me when I was looking for answers:
https://www.thetechgame.com/Forums/t=2795527/ultimate-xbox-360-flashing-guide-lt-30-released.html
There is a list of requirements stated depending on your drive.
As for your question, you do need items such as X360USBPro,
but I don't think you need a probe for the slim.

Not to be negative against Team Xecuter, but you most certainly do NOT need a USB pro. It is 10000 times easier WITH one... but I have flashed both slim and phat consoles with just a VIA 6421 board.

I used the VIA board, along with a power supply from a donor Slim 360 and everything worked fine. As far as using a probe for the Phat drives, I just made my own with a sewing pin and the DVD power cable from the donor. Saved me a lot of money.

That being said. If you plan on doing this a few times, or plan on doing it for friends, then I would definitely say get a USB Pro v2. If your just doing it for yourself and are strapped for cash, you can get away with it. That is of course unless its a 16D4S. Then I would say just get the TX Replacement PCB. The Kamikaze hack just isn't worth attempting.

It depends what kind of chip you use.

Of you have an mxic then you can use a probe, you need a ck3 for the probe

If you have a winbond prepare for a hard, n I mean hard. Kamakaze hack