jdfc, seems wagons are popular at the moment, quite a few people on the forum chasing exactly what you are looking for.
Have you seen the red and white one on ebay with the stripe kit? (search fc station sedan)
This is typically what you would expect from a decent wagon requiring little work to get it on the road.
This particular car has been on ebay before so I'm not sure whether it never sold or the new owner is selling again.
$5000 to $7000 is a fair price for this type of car so the $7000 tag on this one is at the top of the range considering it needs a little rust repair.
Have a look at the 2 tone green wagon also on ebay to see what $1500 will get you! (search fc wagon)
FC wagons don't come up often but certainly more often than FE wagons do! - Being in a club and "in the right place at the right time" can bring benefits in terms of seeing cars which never make it to mainstream advertising.
The main thing to look for when purchasing an FC is rust - try and buy a car with as little rust as possible.
Acceptable mods is a question only the owner can answer. Some in our club drive totally stock cars and wouldn't have it any other way, others drive vastly modified cars and conclude that as long as the body looks stock it is still pretty much original. Shows are the only places where a cars originality is studied so that it can be correctly classified for judging.
Personal choice is what I recommend.
A small point, new floor pans would not be classified as a modification - this is restoration.
The Victorian club state titles are coming up soon and I would recommend you come along to our February meeting (2nd Tuesday of the month at the Bentleigh club in Yawla street Bentleigh at 8pm)
Hope this is of some guidance
Ken