Obvious things to bear in mind are as said the age, components are going to be worn.
One of the biggest problems is they attract people who want a fast car but can't afford to run one, so get bodged and abused.
Bodywork/welding will be a major cost if it needs any, which is very likely at thier age, all the usual rust areas.
Personally I have the attitude of assuming everything you are told by a seller is complete lies, some prefer to get a car that looks good, but not myself.
I got 3 folders full of history, a quick look soon confirmed it was crap, I mean a tranny diesel pump on a cossie for example!!
I just buy as seen on spec, get a discount as it needs arches etc this doing that doing.
I absolutely disagree about the reliability issue, there isn't one so long as looked after, for the age of the car they are very good imo.
I would take someone with you who knows about cossies, even if you have to bung them a few £'s money well spent.
Stage 1 as said around 280 is very quick, stage 3 loads more torque.
The major question should be 2wd or 4wd(tractor!!), sorry couldn't resist, they both have good and bad points, 2wd cheaper to run, 4wd the one you want in the wet, as 2wd really do need careful handling, try to get a drive in each if you can.
Ruuning cost wise is not bad at all, they do not need at all a coilpack conversion, the distributor set up worked fine on all cars for years before coilpacks became popular, and that had nothing to do with performance.
The one you want is a mercury grey with tango/amber rear lights.
tabetha