So Many Details

If you're super strapped for a cash though, and just basically want to keep the snake alive, then your current heat lamp thing will be fine assuming the snake is hardy. My sister in law has kept a ball with nothing but a mesh top and a heat rock for over 10 years now. This is a 2500 gram, 4 and a half foot ball python that has never had any illness. Her only issues so far have been a couple breeding season fasts, but she's currently pounding down a medium rat every 2 weeks.

Since an Acu-Rite has a probe, you only need one (unless you're referring to a back-up) because the actual thermometer sits on the cool side and the probe sits on the hot side.

And depending on what brand of UTH you get, it may come with the little rubber feet. But, yes, those are definitely a must-have so that it gets proper ventilation.

A lot of different set-ups work for a lot of different people but I'd still strongly recommend against using a heat rock. They can get hot enough to burn your snake.

Bigger is NOT better.
The more room they have the more stress they have. How old is your ball python and it's weight would be good too.

An adult ball python would be fine in nothing bigger than a 30 gallon enclosure, a 20 gallon would do too.

It's ok, at least your seeking for assistance. For humidity I suggest just getting a misting bottle and fill it with hot water and misting the enclosure, as well as having the top of the screen lid covered with foil. (the hot water will instantly cool down when its sprayed.)

You want one side to be up to 90-93 degrees, this is the warm side. The cool side should be about 85-88 degrees or so. Two hides, preferably identical on both sides, and a water bowl on the cool side. Make sure the hides are not too big! They should be snug around the animal to where they are touching all sides of the hide at all times. I don't suggest half-logs.
Get an under tank heater, and a rheostat. The rheostat will control the UTH's temperature.

Since you enjoy display tanks as I do too, make sure at least the back of the tank is covered with something dark (black construction paper is fine) and the sides would be good too. This helps security. Your free to use fake plants too and decorate the cage if you'd like. You didn't mention any type of substrate, so I'll just give you a heads up on no pine, cedar, or carefresh. All are dangerous.

:]

First I would go for the thermostat, and at least 1 hiding spot, just so that he /she has at least some security with decent enough temps.