Blowing Smoke

I have a 1989 Town Car with roughly 80K original miles. I recently had some work done, replacing the radiator and most of the hoses. I had just been driving it in town, then a few weeks later we took it out on the interstate. Car started blowing smoke out of the vents and wouldn't accelerate past 45. I park the car first chance and take it in to the shop a day or two later when I was able to (didn't dive it in the mean time). They replace the fuel pump filter, and that seemed to fix the problem, at least as far as not being able to go past 45. I drive the car to work and it's fine. On the trip home, at about 2am when it's 30F outside, smoke starts coming up from the vents and fogging my windows. The mechanics said that the heater core might be bad, and when I called them afterwards, they said it probably was the heater core.

I have hot air when I set the system to heat, and it hasn't smoked much since, just a few wisps now and then. I do know that my power steering fluid is leaking, so I think some of the smoke might be from that.

How likely is it that it's the heater core causing the smoke, and if it is, what sort of price should I expect (assuming going rates for labor) to have it fixed?

Thanks.