As some starter food for thought:

The condition of AGC in the weeks preceeding Typhoon was only fair. But, this did not have to be. The Germans could have done the following with their available resources if a decision was made to put top priority on this operation:
1. Fully replace casualties in the group's divisions. AGC was down about 55,000 men due to casualties. The replacement army was generating about 130,000 men per month. But, Hitler ordered the majority to be used in filling out new units in France giving AGC only about 25,000 replacements. This changes to a full 55,000 to 60,000 men being pushed into AGC bringing it to full strength.
2. The Eisenbahnbautruppen are increased by two additional regiments. This will aleviate most of the rail construction problems in about a month or so. While the rail system will still be inadequite it will not be critically so. This means that sufficent supplies and materials are on hand at between 75 and 90% of requirements, and that winter clothing will be available when needed for the most part.
3. Panzer production is directed to AGC making up for losses almost entirely. Production was about 320 vehicles and would have brough the AGC panzer divisions up to full strength or nearly so.

This means instead of AGC starting Typhoon at an average of 60 to 70% divisional strength with panzer divisions operating at 50 to 60% strength and the associated truck strenght (particularly in the Grossraumtruppen they are closer to 80 or 90%. Instead of having only about 50% to 70% of the necessary supplies, and in particular a shortage of artillery ammunition, the group would again be at about 75% strength and in better shape with reserve supplies.

These changes alone make a huge difference in the starting position of AGC. The Soviets facing them at this point are in really bad shape at this point. Many of their divisions are at or below 50% strength. Many are badly disorganized and lack much of their heavy equipment and weapons. Most of the Soviet tanks are the pathetic T50 /60 /70 lights that are all but worthless as battle vehicles (about 60% of those available). The T34 and KV 1 still represent only about 30% of available AFV and are still being committed piecemeal.
Many of the Soviet replacement divisions and ones from the interior are of poor quality and lack training. The only really bright spot is that a number of cavalry divisions of good quality are available.