Ideally you would use the same truck year-round. That way you are not depending on plowing to justify the expense of a 2nd dedicated plow truck. When I started plowing, I sold my F150 and bought a 2500HD with a plow. So I invested more money, but I was ready to replace the Ford anyway.
And a $5,000, 23 year old truck is not a low risk purchase. I think you may be better off selling your existing truck (assuming that you own one), adding the $5K you were going to spend on the 1989 truck, and get a nice newer truck with plow.
Or...why not buy a 2 or 3 year old plow and put it on your existing truck?