1994 Chrysler LHS Thermistat

My car has been over heating. I have checked the oil, added water and coolant at a50/50 ratio, and have check the fuse for the fan. Everything checks out fine, but it still over heats. The only thing that comes to mind for me is the thermostat is shot. WHen I spoke to a mechanic about this, he told me that I will have to bring it in so that he "use the coolant flush machine to make sure you get all the air out, alsoneed to pressure test it to locate the problem." I did not ask for a price because it will be more money than I can afford right now. My question is simple, do I really need to do all of that? To me I would much rather spend the $20 on a thermostat and install it myself than taking to a mechanic and plugging it into all these machines for him to say it needs a $20 thermostat. What are your thoughts. Even after I install a new thermostat, do I still need to hook it up to a coolant flush machine. I have never needed to do this for any of my other vehicles? Thanks for all your help.