Its quite a major job replacing a head gasket.
The gasket itself is easy to do, but getting to it is a major operation.
The average car will require a full days labour to pull the engine apart, and put it back together again.
Iv done it on a race car in only a few hours, but the average road car has more odds and ends around the cylinder head to remove, and working on a customers car requires a more care and time is taken to ensure the job is done right.
If you can do basic mechanical work, like fixing brakes, changing a cam belt etc, have knowledge of how an engine works, and have access to the right manual, and the right tools (you will need a torque wrench, and know how to use it, as well as a complete set of sockets) then you might be able to do the job in probobly 2 days.
You will need roughly about $100 worth of parts, as there are several gaskets that need replacing, and while your in there you should do things like the water pump and thermostat as well. (espcialy if you have a leaking gasket, as there may have been air bubles in the coolant, which will cause the water pump to cavitate, and damage itself).
Given the age of your car, and your apparent lack of automotive technical knowledge, I would recomend getting your mechanic to do the job for you.
If you are infact mechancialy minded, and have a mate who does know whats thier doing, and you can afford to have the car off the road for up to a week (things may go wrong) then you can certianly save yourself some money doing the job yourself.