Your 1991 system almost certainly uses r-12.
1994 was the first year in which r-134a was mandated in the US, though I understand that some cars had it before the deadline.
I will not be the last to tell you that there are many cost and performance advantages to sticking with the r-12.
The refrigerant itself may run $80 or more, but that is not particularly significant relative to the cost of of a proper conversion, or the cost of A/C components and service overall.
Even if you decide to convert, I do not think a conversion is a good DIY job unless you want to invest a lot of time in the training and research required to do it properly.
Avoid those cheapo DIY 'drop-in' conversion kits. They are bad news.
Best of luck.
1986 300E 5-Speed 240k mi.