Wait a minute..... Let me pop in with another perspective...
Stainless can rust, 304 in particular rusts rather easily, like was used on these early boats. One cannot with a broad gesture simply say if bolts are rusted that they are not stainless steel.
One does not need to melt the lead keel to sister-in new keel bolts either. There are keel re-fastening kits available form numerous places. Basically big lag bolts. The procedure is relatively straight-forward.
For a 70's C27, the bigger issue is the condition of the wood in the keel stub. One needs to get to the wood and determine if it is rotten or soild. The repair to replace the wood is not too difficult, yet tedious and it is well documented here and elsewere. A search will turn up forum threads detailing it.
If the boat is world-class, well maintained, has lots of properly-installed options, one might not need to run from this boat but rather asses the work requred to repair the keel, compared to the asking price.
If however the boat is neglected, tattered sails, requiring lots of work, you might consider walking politely away, or bookmarking it for further consideration of the price can be lowered to make the deal satisfactory to you.
Endless, is there any way you can get pics of the rusty bolts so that other people more knowledgable than me on the topic who have done the repair might chime in?