The decision matrix I give everyone on "keep or buy" is like this:

Do you do your own work? If no, plan on a minimum of $1500 in the first year to have stuff fixed by a pro...might be more in NYC. If yes, plan on about $500 in parts and two weekends of work minimum in the first year.

Does the car have maint records? If no, no matter the price, offer a minimum of 20% less. If yes, go through them meticulously to estimate what needs to be done keeping mind that some things have to be replaced based on age, not mileage.

How many miles a year do you drive? If less than $10K, you are looking at MANY years of service at a slightly higher per year maint cost that is still likely lower than your sum total for initial maint of a "newer" car. If more than 10K per year, the balance can easily shift to the lower mileage car.

Finally, this specific car. The 1992 has a better AC system but otherwise is in no way superior. I would be a willing to bet a used, low mileage tranny installed in your current car would be less than the initial maint cost for a new-to-you car...not that I really believe you need it. You need to flush that tranny using the very basic instructions in the FAQ's. Your 1990 will also be nearly impossible to sell for very much money at 300K miles.