1 ~ Know the Local Zoning and Building Regulations. That charming Conch cottage built back in the 40's certainly carries a lot of nostalgia and Keys history, but your dream of expanding it and turning it into a spacious new home won't be easy. Many of the ground level cottages built in the 40's, 50's and even 60's are built below base flood plain elevation. That will make it difficult, if not totally impossible to add on to it, since you will not be given permits to expand the home at its current elevation. It may also cost more to insure with flood insurance.
2 ~ Price Building Supplies and Contractors You Will Need Locally. There's a saying that Paradise isn't cheap. The cost of building supplies and even labor can be quite different than what you may be used to paying back home. The Keys don't have but two Home Depots that serve the entire island chain from Key Largo to Key West. Many areas of the Florida Keys are still completing repairs from Hurricane Irma, and finding contractors can be challenging and expensive.
3 ~ Don't Skip Inspections Because You'll Be Remodeling Anyway. It may be tempting to skip the inspection period, especially if you're planning on tearing out a kitchen or bathroom. But having an inspection may bring to light issues in areas of the home you weren't planning on changing. You don't want to start taking out a kitchen or bath only to find that the plumbing or electrical systems are inadequate. Having an inspection will look at the whole house, and can help you decide if you plans will even work once you've purchased.
Finding an older home and making it your dream home is possible, but be sure to keep an open mind and work on the project knowing the facts before you purchase.