When buying our 1994 320GT earlier this year we found the road tyres to be about 12 years old, but the spare was original - 25 years old!!

Hi we tend to keep our tyres covered if they're in bright sunshine for extended periods.
We try not to let the tyres go over 5 years old, 6 the limit especially if they are stationary for long periods.
Also check the load rating of the new tyres. It's given on the side wall with max pressure allowed figure. From this it's easy to work out the pressure you need.
Generally commercial tyres fit the bill for load but check the speed rating.
We've used 'Tyres on the Drive' and found them excellent.
Best regards
Tom