No an aftermarket boost gauge is not normal as that car should already have a boost gauge in the rs monitor already. I don't think it should be a deal breaker though as long as it has its service up to date and books to prove it.
If you want, you can ask the owner to get it inspected by your chosen mechanic to get it looked over thoroughly (obviously you pay for it).
These cars are generally 'bulletproof' mechanically.
Good luck with car shopping