Does this sound like a reasonable way to slow cook a rib-eye? OK, I will shortly be cooking my very first large piece of meat (i.e. not a single steak, but rather a larger one that I will carve into pieces when serving). I think I have the procedure down, but I want to go over it with you guys to make sure that it sounds good.

The steak is just over one kilogram. I will remove it from the refrigerator one hour before cooking.
I am planning on searing the outside, then placing it in a roasting pan on top of carrots, onions and celery. I will use a 120 degree (celsius) oven, on the convection setting. I will be using a temperature probe, placed into the centre mass of the steak, and shooting for a target temperature of 53 degrees celsius. 

I am expecting a post-roasting rise in temperature of 5-7 degrees, which should leave me with a nice medium rare steak. I will rest the steak on the cutting board under aluminium foil for at least fifteen minutes.

I am also planning on deglacing the pan with a mix of 50/50 red wine and tap water, then making a gravy based on the pan juice and a basic roux.

Does that sound good to you guys?

I have a choice between using a roasting pan and a cast-iron pot. Which should I use?
The method sounds fairly traditional.

You may find better results doing the oven phase first, and the searing phase last.  By doing so, you will desiccate the outside of the steak, making it begin to sear more quickly (since there is little or no water left at the surface to evaporate).

The pan you use for the oven phase matters very little; cast iron will be superior for the searing phase.

Since you are going for a fairly classic red wine pan sauce, you may wish to investigate butter mounted sauces, rather than doing a roux based sauce.  This may give more intense flavor, but is certainly not core to the technique for the steak.  This Chicago Tribune article has a good summary of the technique.

The one key to success that you have left out is seasoning the steak.  Perhaps the best method is to season it liberally the night before (or at minimum, 40 minutes before cooking), and allow it to sit overnight in the refrigerator.  At first the salt will bring liquid to the surface of the steak, but later it will be reabsorbed, spreading seasoning into the meat itself.  See Kenji Alt's Food Lab article for more details.