If this car has been very well maintained then I would say in all likelihood it will be a solid buy. The 06 Altima is the 4th generation of this line. It very likely has the CVT for its transmission. The 2.5 CVTs have been more problematic than the 3.5s. Would seem to be contrary to what one would surmise based upon the factors involved. A higher HP and torque transmission would seemingly be a worse fit for a CVT. However it appears that the 2.5 VQ seems to not be such a good pairing with the CVT design. I am hypothesizing that it's related to the fact that the 3.5 VQ keeps the transmission in a better operating state with a CVT. The 2.5 VQ just doesn't have enough power to keep the cars in the better rpm zone versus the 3.5s. I also strongly believe that the other problem has been people using the WRONG CVT fluids when they have changed their CVT fluid. It really should be the Nissan NS-2 CVT fluid only. If there have been issues with the CVTs they typically occur MUCH earlier in the cars history. This car having 157k plus miles without any trouble would be a positive sign. My 08 Altima 3.5 SE has been absolutely fantastic so far. I enjoy driving it a lot.
Far has oil goes I would suggest a full synthetic group 3 at minimum. The heat in these motors is rather high due to the cover over top the motor and a bit of a small oil capacity. My 3.5 has a 4.5 qt capacity. Well my 3.0 Ford Fusion had a 5.5 qt capacity. Which does help give the oil a bit more time to "cool" down time while out of circulation in the motor. At the end of the day really any solid synthetic oil will do the Altima fine. Whether that be Pennzoil Platinum, QS Ultimate Durability, Valvoline Synpower, Mobil Super Synthetic, Castrol Edge etc etc. The oil change interval is 3750 miles for severe service or 7500 for mostly highway miles. The VQ motors shear the oil in them rather quickly from a 30 viscosity to a 20 operating viscosity. High heat is another factor too obviously.
I would also get under the car and check for rust, also CV joints/ boots for cracks or tears, any fluids leaking. Tire condition as well matters because if the car needs 4 new tires I would subtract that from the price. Check tire wear. If the tires are wearing irregularly than the car is out of alignment. Take that out of the price has well. Has the coolant been changed as well? Check the Drive belts condition as well. I hope this car will be good for you as my 95 Sentra was when I got it with 118k miles.