I'm thinking about buying a 2002 S60 T5 with 78k and a manual transmission and then selling my 1999 S80 which has 53k and an automatic transmission. Are there any areas of concern to check for on an S60?

 

Have the car checked out by qualified Volvo technician. It depends on what work has been done recently. Aside from the normal tires and brakes that is normal to any car, by that mileage, almost all S60's will need or have already had:
Front wheel bearings - if you hear a humming sound beginning at ~ 30mph that changes tone when you go around turns.
Front control arm bushings - a clunking sound when the car goes over bumps. It's easier to change the whole control arm than replace the bushings.
Sway bar links - easy to change
Tie rod ends - possibly inner tie rods, too
Maybe struts and shocks, along with their mounts.
I'm seeing some people have had to replace the steering rack, myself included. I've never experienced a worn out rack before.
Turbo control valve
I can't comment on the clutch life.
Also, check if the window controls clicks when the car has sat out in the cold. The clicking will disappear as the interior of the car warms up. It is a sign that the controller is failing. Eventually the controller fails and you can't operate the windows or mirrors. That's a $600 fix at the Volvo dealer because the unit needs it firmware synched to the car.
Do you do your own work? Everything is relatively easy except a steering rack and a clutch. The wheel bearings, tie rods, control arms and sway bar links will cost about $2k+ at a Volvo dealer.