Hiya John! Welcome back!

You have all the experience you need to work on one of these. They're DIY friendly enough. Though the oil filter change is messier than older white blocks and changing the cabin filter is ridiculous.

Like many others, the o-rings on the heater core connections are prone to failure over time, causing coolant leaks into the cabin. The front sunroof drains are also prone to clogging so check for evidence of water/coolant on the carpets (it gathers in the back seat floor).

The right upper engine mounts fail quicker than they should.

Check the headliner; they fall eventually. Big job to refurbish. Another bargaining chip.

Replacement keys are pricey, if this one only comes with one you should factor that into your offer.

And finally, if you do buy it, please inspect the coolant hoses and, if original, replace PN32 in this diagram at your earliest convenience: https://www.volvopartswebstore.com/images/parts/volvo/fullsize/GR-380045.jpg
Many a S40 has gone to the scrapyard prematurely after the plastic Tee in that hose failed.

Best of luck to you! They're nice little cars.

--Will