The issue if you do choose to convert to another refrigerant is that you should change AC hoses. As they age, they crack and develop leaks. Often old R12 systems will not leak as bad as R134A, due to the accumulated oil and the larger size of the R12 molecule. It is often the case that an old R12 system that leaks down over the period of a year will do so much more rapidly if converted to R134A. Freeze 12 is simply R134A, with a portion of R142 IIRC to help it be more compatible with the oil in the R12 system.
Converting to R134A can be a mixed bag, in that some people have great success simply vacuuming and recharging with R134A, while others find no satisfaction. If you have a good AC shop, and plan to keep the Rover for a LONG time, converting can be successfully done and save money over constantly recharging with R12. That option could cost near $1000, if you choose to "go all the way", but if $150 is the going rate in your area to recharge with R12, it would pay for itself over a few years. By the way, you are not actually supposed to recharge with R12 without repairing the leak. Releasing R12 into the atmosphere is not exactly legal or environmentally friendly. By the same token, while not destructive to the ozone layer, R134A is a greenhouse gas, and is relatively toxic if you breath too much.