Sorry for not replying sooner.
Place's to look for rust:
Windscreen surround, If you watch when it rains the water that lands on the wipers will run down the edge's & off the side of the car or down between the bonnet & wing, & on to the sill, water can also run into teh car if the doorseals are not good CHECK ALL THESE AREA'S
Around the battery & check the drain holes into the wings.
The rear windows have metal tray's to carry water down drain pipes and out the bottom of the sills, make sure this pipe is clear & the metal tray's are rust free, check the window winder's are not rusting. this car cause the sill to rust from the inside out. Removing all the interior (seat's carpet side cards) will let you check the whole car.
Check the bottom of the door's & make sure drain hole's are clear. Cheek hinge's
Check the mounting points for the bootlid strut's. The rear suspension turrets can rust, due to dirt from the bottom of the car holding moisture. Dirt also get's trapped around the fuel filler. Remove the wheel & clear out with your hand.
Check fuel tank for signs of rust. if it's still ok remove tank & rust protect it.
Spare tyre well can trap water which will rust. Remove all carpet from boot & have a good look around the side's.
Unfortunatly Sprint's seem to rust everywhere. Clear the dirt out of every part of the car, dirt holds moisture!
If I was you & had a rust free Sprint I would get a specialist rust protect the car, It would mean striping the car completly, but if you had teh space it would allow you to rebuild better than it was before, plus you would then have an intermit knowledge of you car.
2x 145, 4x 75, 4x Sprints,a 156, 146 now a 155