You can certainly pull the heads off, check and make sure the deck is flat, and check the 4 corners to see if you need to have the deck cut; if you do, this may be a deal killer for you.
At this point you can also see how close to the top of the bore the piston comes up, and what kind of valve reliefs, domes or cups the pistons have...you may also get lucky and find a part number which would help immensely.
If the pistons aren't close to the deck; its going to be hard to get any efficiency out of this, no matter how much you mill the heads. You can play games with the head gasket, but if the deck isn't prepped properly you may have trouble with a shim gasket.
Once you know the state of your deck & pistons...now you can make an informed decision about milling the heads, larger valves to get the compression you like and the breathing you need for your performance goals. The camshaft should be the last piece of the puzzle.
BUT a towing rig, an off road truck, a street brawler or a daily cruiser...all have different requirements...start there