how hard would it be to replace them ?, is there any special tool required ?

assuming they're hard to change, if i were to pay a shop to replace them, how much money would i be looking to pay more or less ?

 

They are not hard to replace, but you have to remove the rockers and valve springs. If you are not familiar with this process, best have a shop do it.

 

I did it last month. It takes a good supply of compressed air in the cylinder to hold the valves up. Take the spark plug out, and buy a (compression tester type) hose to plug in. Then you need a spring compressor. I made one from a piece of channel.
With the right length of bolt it works. It takes time to carefully remove and replace each seal. While you are there, throw on some new springs.

 

I haven't done it yet, but bought the little tool to help install the new ones. Prob a waste of money, but it's in the tool box now.