Good questions. To be honest I'd suggest taking a look at the car to see what sort of conditions it's in. Probably a good idea to try and see some other fibreglass based cars as well if you're in the market for one, see what you think... perhaps find a club meet / event near you to go along to??
From general experience though...
What's a fibre glass car going to be like to drive? -
Depends on the car beneath... although fibreglass is pretty lightweight so should be lighter and therefore a bit nippier than a metal bodied equivalent. As for handling etc it depends on the car itself.
Will it be obvious its not a metal car? -
Depends on the finish and whose looking at it (i.e. if they know what they're looking at).
How safe is a fibre glass car? -
Fibreglas cars pass the SVA / IVA all the time so they're not inherently dangerous. Although it's definitely worth noting that during an impact fibreglass will splinter and shatter rather than dent... so you'd be relying on the strength of the kit's chassis above all else (and replacing / repairing damaged panels rather than just beating out dents!).
How does a fibre glass car weather with age compared to sheet metal? -
Again it depends on the finish. Fibreglass can be known to fade and blemish with age, specially in areas of regular flex or movement (bonnet panels etc), but don't forget that you'd also experience degradation with a sheet metal finish. As above thought, if things do go wrong then you'll end up patching up or replacing fibreglass sheets rather than beating panels.
It all depends on what you like. Personally, a fibreglass body wouldn't put me of a kit in general. The only time it would is if it really didn't suit the kit itself.
Hope that helps!