I'd be inclined to say if it ain't broke don't fix it.

I would say that if you are unsure of when was the last time the timing belt and pulleys were changed then its time to have those done.

I'm also a fan of fluid changes...coolant, brake fluid, power steering, etc.

If it were me I would wait until...and only if....a component fails that you start replacing. I took the plunge about a month ago, had a 98 XC70 with about 215k on it. Few things needed attention, AC Evap repair @ $800 or more, alternator and control arms needed help....few additional maintenance items. All in all I was looking at throwing $2100 give or take into it and could have been trouble free for years to come...but instead I picked up a used 07 S60. The XC was a beauty inside and out, little wear on anything. Driving 500+ miles a week I opted to go back to FWD and yield about 29 MPG on the highway as opposed to 23 ish I was getting.

Regarding your cars, even though 180k and 190k sound like a lot I'd stick it out and tend to the small things that come up...whilst abiding by all maintenance items as necessary. As you are familiar with the older red block motors I am sure you know you can rack up the miles on these cars....with proper maintenance that is (on the 5 cylinders). - Why do I say this? Well I am still wondering if my upgrade choice was the best idea. Don't get me wrong the S60 is way nicer, but, 2100k of repairs vs car payments for quite some time....well I guess its a choice. I will surely miss the AWD performance of the XC70 with its complimenting snow tires. On many of occasions I tried to get stuck and couldn't, it almost became a game.

Steve