The wagons tend to have a problem with battery acid running down the firewall and eating all the way through... so I would plan on really addressing whatever damage has occured and prep for the future.... like with POR15 applied per directions... and get one of those sandblast/suction deals so you can really clean the area a few square inches at a time without getting sand all over the rest of the engine and car...
When changing all those fluids... when the block is empty I would take out all the freeze plugs and power wash with a 90 degree head all the block insides you can reach from the core plug holes... I have seen cars where antifreeze crystalized and caused the coolant to not circulate all over....and then you can always just have muck and normal rust accumulate in the ( particularly rear ) lower pockets ...keep cleaning until all you can get to come out is drinking clean water....
If you happen to take the engine out... then you can address the oil supply channels which are sealed with steel balls or screw in plugs at the rear of the engine...
These are the types of places which would get cleaned naturally when a block is vat cleaned...in a major overhaul...