vivanov1 - I buy my parts at www.bavauto.com (I'm not in any way associated with them).

There's a DIY somewhere on this site about replacing the thermostat.
To replace the thermostat you'll remove the radiator although there have been some posts about doing the job without removing the radiator. Having done the job myself I fail to see how it's possible unless you are Harry Potter.

You will need a 32mm wrench to remove the fan. There is also a special tool to hold the housing in place while you remove the fan. I highly recomend you buy the tool. Hayaku has a great DIY on this site I think.

All that said, if you go to all the trouble to replace the thermo, you might as well replace the water pump while you have it all apart and replace the hoses for the radiator. Amount of fluid varies because there is no easy way to "flush" the radiator that I've found.