Ashley would almost definitely be able to fund a 30m purchase, NUFC would be able to stretch themselves to a 30m player if they thought he was worth it. Newcastle's problem is that virtually no 30m player would go to them.
I think Ashley is after Rangers because they are at present a basket case that is struggling but there is still the infrastructure and fan base to give someone huge profits if they sort out the problems. While the CL will be a strong incentive for Rangers, he'd actually get a better return by getting Newcastle to the CL because TV rights are better, they have a bigger ground and I'd assume the ticket prices are much higher. The problem at Newcastle is that the competition is too intense in the PL and they'd have to spend 10s of millions and still probably end up missing out.