Either Martin M or Speedy Spares should do the thermostat.
An upgrade to a leccy fan is well worth it, even better if you can mount it behind the rad. I fitted one off a turbo diesel Escort and it works brilliantly, I had to move the rad forward and make up some bracketry but it was worth it. Of course you can get one straight off the shelf, why not try Ratsport - Commerjules runs the place and has used one on his van successfully and that's running a Holbay lump Just be aware though that occasionally they don't seem to work very well on some engines:I
I made up a new thermostat housing and included a connection for a switch but you can get a type that fits inside the top hose with a special seal - or fit a 'T' piece in the hose to take a switch.
Mine is just on a temperature switch, I think it's a Fiat one (just happened to be the right temperature range and it was less than a fiver) but Kenlowe and others do an adjustable controller - but not cheap.
I have found the upgrade worth it, unexpected bonus was quieter cabin at motorway speed (engine fan is quite noisy at speed but electric fan probably wont be on at all at speed)
Controller here: http://www.ratsport.com/PBSCProduct.asp?ItmID=9947274