If you want it automated you should buy something like the thermostat wizard is selling. His stuff is good quality and prices reasonable. I dont have one of those but bought his breather kit a few years back. There are other ways, my fan is rigged through a relay to a switch on the dash. I am confident that I check the temp guages often enough to flick the fan on when I need it.
If you want a fit and forget option I would buy wizards. Wizard - put the fiver in the post.... :D:D:D:D ;)
I have gone for the switch option on my 110 tdi conversion as well because I am a tight sod and it really was done on a shoestring budget. However, a thermostat is something I keep saying I will buy.....