Um, how much is the seller asking for a 60? It would have to be a pretty serious discount for any battery upgrade plans to make economic sense.
People don't replace the batteries on Teslas (unless you are wk057 or some total ninja Tesla hacker). A new battery from Tesla would cost about the same as the car is worth:
New batteries are like $25k, plus $15k more if you don't let them keep the old one.
Perhaps you are concerned with battery degradation and you are assuming replacing batteries on Teslas is or will be a common occurrence. It's not. Most Tesla batteries, even ones approaching the end of their eight year warranty, still have 80% or more of their original range.
One thing is for sure, Tesla won't be upgrading you from a 60 to a 90. I don't think that is an 'official' option ever.
The good news is no 60kwh pack was ever installed in a Model X no matter what the badge says. All X's originally limited to 60kwh actually contained a physical 75kwh pack: the limitation was purely in software.
So if you have a 60 that is still locked, you actually already have a 75, you just need to convince Tesla to sell you the software "upgrade" and magically turn it into a 75. No new hardware is required, just a credit card. Officially they have stopped offering this, but there are some (relatively recent) reports of persistent owners having success with this. That might be a legitimate upgrade strategy?
(Side note, if you keep it locked to 60, at least you never have to worry about charging it to "100%" : since you would never be able to charge it to more than ~80% of its capacity no matter what the software reports, you could just max it out on every charge.)
In general it sounds like you are concerned with range and you imagine a world where you save money by knowingly buying a car with a limited range and then somehow improving it later. You should avoid taking a car that currently only has 60 kwh of usable range unless someone is giving it to you for free. It will be far more cost effective to just get a car that already has the range you need. No one upgrades the physical batteries on Teslas.