Wow that Classidur stuff sure is pricey. If it works as well as footprint says though, it'd be money well spent. I was interested to read about it.
I've used Zinsser BIN bulls eye 123. It's water based and although it has looked like it hasn't worked very well, when you emulsion on top, you see it does actually work. It's half the price of the Classidur so still not cheap, but cheaper..
I've used oil undercoat and it will work but for me personally it's stinky, not all that cheap, you'll do in a roller putting it on and you might need two coats of it and still two coats of emulsion...
I'd say it all depends on the amount you want to spend, the area needing done and the time and effort you want to put in.