Where are you styx? (ie country, area)
Basically check the condition of the body for rust and damage, if you are not sure what to look for try to take someone with more knowledge with you such as a panel beater. Being 30 years old it is pointless to say look in a particular area, rather look everywhere! Having said that a good guide is to find out what rust repair sections are available - they are available for a reason. Although major rust in complicated sections where no repair panels are available could be harder to fix.
By comparison the interior is far less complicated, although big bits of plastic can be hard to replace (eg dash). Also the mechanicals while important are at least straightforward, and most likely will need a complete rebuild anyway, so allow for that.
I would estimate fuel consumption would be 10L/100km or a bit less (just an estimate though)
Here is a webpage that may help you:
Capri Links Page