Welcome aboard.
I hope your dog can cope, that's a mighty big boy for a
small boat. One time, in a foreign country, the quarantine officers brought their Alsatian sniffer dog-in-training aboard. He probably only went about 100-120 lbs. They sort of chucked him down the
companionway, and he was too big to be able to turn all the way around. He became quite frightened, and they couldn't get him to work. It was really sad, and not the dog's fault at all.
Be aware that you will have to deal with his poops, too, so you'll need a reasonably good sized
holding tank.
I'm not sure where the numbers are coming from (could be only guesswork, I simply don't know) but people are saying to have at least 30% of
purchase price of boat above the cost of the boat for the necessary improvements and
repairs that will be needed for the previously owned boat that you will buy for your
budget.
We see a lot of people come here saying they want to go sailing and cruising. Maybe while you're still working, you could make or buy a sailing
dinghy, and start
learning how to sail it. You do not know, yet whether you will like sailing, although, if you do, you will probably find the folks who do it to be congenial.
Finally, a caveat, if you take your pickup and go camping with your dog, you cannot drown if you fall off your sleeping bag, but you can, with a boat. Some people do not like being out of sight of land. If you're more than 5 miles
offshore in the ocean and off the boat, you are unlikely to swim safely to land....and of course there's the dog to consider.
My point is that you are thinking about entering an
environment with which you are unfamiliar, and it will likely show you some unanticipated hazards.
I'd recommend
crewing on other people's boats, too, because you learn a lot from being exposed to different boats and skippers.
Good luck with your plans.