Cracked Frame

I went today and looked at a 76 Ram 300 Power Wagon(per the hood) Dually, with an International Diesel Conversion. It's a Fire Dept. Mini Pumper that they have up for sale. I was looking it over and the guy said it had a crack in the frame. I looked at it and a crack goes about half way up the frame rail on the side I could see easily. I was wondering how hard it would be to fix this problem... I was thinking you could just clean it up with a grinder, and weld it if it is only the one part. If not Cut a plate out of plate steel and weld a plate over a section of the frame. I am trying to figure out what kind of money I should pay or shouldn't pay for this truck. It is sweet less the cracked frame. Thanks for your comments in advance.

Thanks,
Brandon