Sorry to say it, but I think you have a 0 chance of finding this on any receiver these days, regardless of price. I haven't even seen an AC-3 RF input on $5000-10000 devices. Haven't ever seen anything higher than that, but I don't think you'll find it there either.
Since no LD player can output AC-3 via a coax or optical output (AC-3 is stuff into the right analog channel, meaning that it's very different from how it's stored on DVDs), you're going to have to find an RF demodulator. I'm pretty sure this is what you need:http://cgi.ebay.com/ws/eBayISAPI.dll...2BSI%26otn%3D2http://cgi.ebay.com/ws/eBayISAPI.dll...2BSI%26otn%3D2
As you can see, they are rather pricey. Your other options are:
1) Find someone who can modify the player to include an RF demodulator and allow the AC-3 bitstream output via coax or optical. Finding someone will be challenge enough, but it's quite certain that this will cost far, far, far more than it's really worth. You might be better off trying to buy a modded player from someone who doesn't want theirs anymore.
2) Use 2-channel analog outputs and just your receiver mix it as desired. This is the cheapest option, and if you use PLIIx on the two channel mix, what you get will be pretty close to the original 5.1.