Toyota Repair: Retrofitting AC-Toyota Truck, r 134 freon, r 12 freon

Question
I recently inherited a 1994 Toyota Pick-up with 23,000 original miles.  I want to retrofit it with A/C but am not sure which way to go-with a after-market kit or original Toyota parts as available from the dealer and junkyards.  What are the pros and cons? My mechanic leans towards using Toyota parts.  Is this a difficult job?  Thanks.

Answer
Most larger auto parts stores have a changeover kit, some even come with freon, they are normally less than $100, the kit consists of two adapter valves and some o-rings which you can throw away it is not necessary to change any o-rings unless there is a leak.
You can use the adapter valves from toyota but the aftermarket ones work just as well, it's up to you.
All the old R-12 freon has to be removed from the system, this requires an a/c machine, after the valves are installed it's a good idea to vacuum the system and then put in the new R-134 freon. You could do it all yourself except for the freon removal.