Well other than the dropped valve seat issue the SPI is one of the most reliable motors... fixing the drop seat is not a costly thing and will keep that motor ticking forever.
As far as age goes, age does more damage up in the rust/snow belt due to cold weather because it if very tough on the wire and plastic parts going from less then 32 degrees back up to 150... On top of that because we have to deal with ice, we put salt on the road, salt alters the tempiture that water freezes so that it is much less likely to freeze to the road. The down side is that salt eats paint and corrodes(rusts) metal. So cars up here have a rough life if people don't care for them... Of course a well cleaned and heated garage kept car will have a better life, and can defenently extend the life of a car up here...
I think the real question is, wheater you want a new car or not... And do you feel safe with what you have now...
2000 Ford Focus US Spec Zetec 5 speed "Blue Dragon"
2000 Renault Clio 1.4 DOHC 5speed "Snap Dragon"
Hatch Nation #32