Rust blistering on o/s sill?

[georgeo]

Please could someone tell me how much of a problem Rust blistering on o/s sill is? as I've been looking at a 98 R reg. which looked a reasonable car with the exception of blistering around the osr wheel arch and under the sill level with the back of the door? eg. is this a majour or minor problem on the MX5?