Is this a boat you are planning on using as a trailer sailor? If so I would strongly suggest staying with the 25. The 270 will be such a hassle to trailer sail, that I think it will end up sitting on the hard.
The standard deal in an offer is to make it contingent upon a sea-trial and survey. If either is a problem, you can back out.
"Neglected a bit" could mean a lot of things. If there are any major issues, your surveyor should be able to flag them. Since a survey is expensive, go see the boat first. Look for any signs of water damage. Poke any wood you see to make sure there is no dry-rot. This is especially true in the cabin below where the shrouds attach to the deck (the chain plates).
By far the most important thing to remember is this:
considering upgrading from a C-25 to a C-270 (my dream boat)
How often do you get this chance at your dream boatg 6 hours from your house? Be cautious, but go for it!