There is active road paving from the border post to the border. We took 3 hours from Copiapo to the Murray hut, plus 30 minutes at the border post (not Copiapo) to check the DIFROL permit and fill another climbing form. The border facilities are closed after 5 pm, but if you tell the carabinero that you are only going to Ojos del Salado, he should let you go. When you come back, you can skip the queue if you say you were not in Argentina.
The path from the Murray hut to the Atacama hut splits at some point. The sandy (left) path is more used, but our rented Chevrolet D-Max got stuck as we discovered its 4x4 did not work. After getting unstuck with the help of other climbers, we were able to get to the Atacama hut on 4x2 on the rocky (right) path. So a 4x4 is not absolutely necessary to Atacama, but you do want it for peace of mind and to get to Tejos. We took 45 minutes from Atacama to Murray on the way down.
We took about 90 liters of gas from Copiapo to the Atacama hut and back, plus a detour to Laguna Verde. Book your rental at least a week in advance in summer.
Two new huts were built in late 2017. Apacheta is next to Atacama and sleeps 6-8 tightly. We were told we cannot sleep in the Atacama hut. Amistad is 400m off-route on climber's right at 6090m or 725m SW of the end of the car path, from what I see on Google Earth. I do not know the trail leading there and no group used it when I was there. I think its construction was more a show of force by a truck company than real use to mountaineers.
There is a footpath and a car path going up from the Atacama hut. If walking, the car path is easier (but not necessarily faster) on the way up and the footpath is faster on the way down. There is a 2nd split not much later and the car path is the better choice for the up and down.
On the summit block, I found the rock and fixed rope quality to be rather good, but your mileage may vary.
A handled ascender made the summit block a breeze for me, but it's overkill. The western guided groups I saw all had a harness and some kind of friction knot or device, but the local groups went "free solo". I brought a helmet on the account of rotten rock but saw no rock fall from the passage of 10 people and no one else with a helmet. No one brought rope.
Guide companies will ask for double boots, but I agree that you wrote "very warm shoes". I brought double boots to base camp but decided to summit without them. There is no need for the extra weight of a mountaineering boot if you have hiking footwear of enough warmth.
Canister gas available at Lider and Tottus in Copiapo.