if Peru is a country of mountainous and traffic inconvenience,Leopard II may not suitable for Peru, such 60ton monster in Peru maybe only stay in the capital city of Peru, can't go anywhere. Leopard II suitable fore Chile may not suitable for Peru. of caurse the price is one of the consideration, VIT-1A price is cheap, but performance not bad, it get much morden upgrade,with 125mm gun. if Peru not try to enter Chile for battle, but just defence it's self in Peru, I can't think any reason that Peru is not going to buy VIT-1A.
as you know Leopard II,M1 as cold war product,suitable for operations in Europe,how many Asia and central,south America countries to buy it,Pakistan face to India, still buying BMT2000, and India do not buy Europe and the U.S tanks, India is fully capable to buy.South Korea face to war any time, it's future tank K2 cost USD over 7million but its weight only 55ton. Israel's Merkava tanks, the initial design of its surprising, but it is very suitable for Israel.
China type99 weights about 51ton, China is fully capable of its tanks, the weight increased to 60 tons, but they don't, why?
I think Chile's terrain is similar to that of Peru. And if Chile can use Leo 2, I believe Peru could too (not suggesting that they do though). As to more modern upgrade, I don't think Chinese tank had more modern upgrade than Leo 2. and a main gun is not always measured by the calibre of the rounds fired, but they the kinetic energy developed.
Also we don't just look at the weigh of a tank as a whole, more importantly we must consider the weigh distribution of the tank.
If the tank is designed to be more agile so as to travel in dense forest, then the following must be considered -
1) Size
2) the power the engine produce
3) The speed of the tank
Also we don't look at tank at their initial cost only but also consider maintenance cost, ease of maintenance, operating cost (fuel, etc).
Finally if you really want a tank to travel easily in forested and mountaineous area, you might as well buy a light tank rather than a MBT.
So the reason for Peru to buy chinese tank (if they really wanted to buy it) boils down on the following,
1) What their neighbour has and was operating.
2) initial cost of the tank
3) operating cost of the tank
4) availability of spare parts or ease of making and building of spare parts.
5) size of the tank
6) the army's preferrence (boils down to individual drivers' preferrence.
7) Logistic
Also in your enquiries of why chinese do not increase the weigh of their MBT to 60 tons, the explanation could be as simple as - there really is no need to. They might have better materials, they think it is not needed because heavier tank consume more fuel and might be slower.
It actually boils down to the tactics that the chinese employed and what they think is important.