97 b2300 ac blows HOT air

I am looking into buying a 97 b2300 with 136,000 on it, but when I test drove it the ac blew hot air, the guy says it works at full blast when going about 55, and it just needs a recharge to work right again, because he thinks there might be a leak. First, what kind of refrigerant does this truck take? And second, if I have to diagnose a leak or replace ac parts, how difficult\costly is this going to be? Will it be worth it to buy this truck? Thanks