Danger, Will Robinson!
First - 400Es are great cars. One of the finest used MBs. You get 85% of the capability of a 500E for less than half the price. This gets a big
from me. A 400E will also return the same fuel mileage as a 300E/E320 as long as you drive somewhat conservatively. Drive it like you stole it (ahem) and the mileage will drop, as should be expected.
My warning is because you mention leaky A/C. I don't understand these sellers. "A/C needs a charge."
Well if all the A/C needs is a charge then why not give it a bloomin' charge so that it works properly when you're trying to sell the car??
Anyhow, Mercedes A/C systems almost N E V E R "just need a charge." A 400E A/C compressor is very expensive. That could be the problem. Even worse it could have a leaky evap. If the evap is leaking, immediately knock $4K off the price you're willing to pay. That project is $2,800 of shop labor just to do the evap as the entire dashboard has to be removed
and replaced
.
Get a thorough, independent diagnosis of why the A/C is not working. - you need to understand this before you proceed.
The car will need the wiring harness replaced if it has not already been done. This is probably the only part where the 400E part is cheaper than the 300E part, I don't know why.
The M119 motor is really stuffed in there and these cars tend to run a bit hot. If the cooling system isn't up to snuff, watch out. These are amazing motors and the only way to kill one is to run it hot or run it w/o oil. Radiator replacement isn't hard. Fan clutch is shockingly expensive for just the part. (>$500!!) Aux fans need to be working - if they aren't, the culprit is usually a big resistor right behind the driver's side headlight that rusts. It's a $30 part and takes 30 mins to swap out - not a big deal.
I would avoid a 400E w/ slip control. The system is complicated and troublesome.
Report back w/ results, and good luck.