Toilet replacement help - rusty flange?

I convinced my folks to replace the original round bowl cheapo toilets that are original to the house which was built in 1986.

Getting the old toilet out was super easy. I'm now cleaning up the old wax seal in preparation to put the new one in. However, when I was cleaning the old one off, I found the cast iron(?) flange was rusty.

Is this a problem? Should I replace it before I put the new toilet in? 

I went through the rodeo in our guest bathroom last year. Looks like some moisture got to your plywood. I'd do some poking at the subfloor and make sure it's still sound.

They make drop in pvc flanges, that's how I went after replacing 3ft x 3ft of subfloor and retiling the floor.

Also, wax sucks, the foam rubber saved me after two wax fails 

In reply to Apexcarver :

From my poking around, the subfloor seems sound. My dad replaced the linoleum about 10 years ago and he said it looked good then too.

Oh, and I did get the Fluidmaster rubber flange gasket kit because berk this gooey wax E36 M3

Replace it.  If the toilet is not secured, it will move and eventually leak.  This might be what you need.  Used one on my son's '50s house with the same issue, worked perfect.  A strong second on the foam seal instead of wax.

https://www.ebay.com/itm/163660491554?chn=ps&norover=1&mkevt=1&mkrid=711-213727-13078-0&mkcid=2&itemid=163660491554&targetid=4581183927179145&device=c&mktype=&googleloc=&poi=&campaignid=418233787&mkgroupid=1241348861725295&rlsatarget=pla-4581183927179145&abcId=9300542&merchantid=51291&msclkid=e7a6d38c990a14ff7030b359b79ff656

Wax works great. Just use two. Replacement flanges are a nearly daily occurrence for most residential service plumbers. Wood subfloor is the best/easiest to work with so no worries there. I like the one piece repair rings vs. the split jobs like above, but whatever is cheaper or available works fine. 
Also, please caulk the toilet after installation (and after checking for a good seal by flushing a few times). It helps keep the toilet from moving and disturbing the seal you just replaced. 

In reply to barefootskater (Shaun) :

After I cleaned some of the rust away, I felt like there was enough material left to just go with it. I used the rubber/foam seal and the new brass studs. The subfloor is a little warped due to years of having water splashed on it from the bathtub, so the toilet rocked front to back a bit. I used a paint stirring stick as a shim under the front of the toilet which took all of the movement out. I checked for leaks probably 10 times. Everything came out good. I'll see if I can caulk the base tomorrow.