engines are pretty reliable, as long as it's a runner then you should be fine.
gearbox will crunch into 2nd gear, if it doesn't then you've got a rare car
brakes, depends on how old a sprint it is. if it has outboard, 33 based ones then they are no problem. if it is a metal bumpered one then it will have inboard fronts, with the hand-brake at that end, from a sud and they can be problamatic. but not a deal loser.
rust as you say is your MAJOR concern.
jacking points, sills, floors, wing rails, inner front wings etc all rust and are expensive to repair if you are paying someone to do the work. if you plan on doing it yourself then be prepared to have 3 or 4 times the amount of welding to do as what you can see.
don't ignore things like front/rear panels, wheel arches, doors etc
decent 2nd hand wings are hard to find, forget new ones unless you have a huge budget.
front screen surrounds rust around the bottom corners and again are costly to repair if paying.
"C" pillars rust under where the tailgate shuts. rear quarters rust along the bottom edge of the window.
while on the subject of rear windows they wind down slightly from inside, at one end. if they don't then the mechanism is siezed, can usually be freed off but will most likely mean the drains are blocked which will have rotted out the water traps and any rain water will go into the sills.
simple check to see if the drains are clear - pour some water over the front edge of the side windows and look to see if the water comes out the drain pipes inside the rear arches just infront of the rear wheels. if it does then chances are the colelcters are good, and water isn't going into the sills and rusting them.
if water doesn't come out the drains then figure on having a lot of rot to repair in the sills/floor at the back.
interiors are very hard to find so don't ignore worn seats thinking a decent set can be found without a lot of searching.
but having said all that once you hear the car start you will buy it anyway
whereabouts is the car anyway? anywhere near me or any other owners from here? another impartial pair of eyes never hurts