Which one is better performance and (approach): store as rows or as a varchar column

The only thing I'll add is that I find it easier to split later than combine later. Depending on what type of information you are looking at about Boats or Cars and how many points you care about and what you are doing with the data (To @JackDouglas's point about needing more specifics to give specific advice) maybe you won't need them combined, maybe you will. If you aren't sure and can design with them combined and are leaning that way, it is worth a try and testing knowing that someday you may have to split them out. Won't make refactoring easy, but IMO it would be easi*er* than trying to combine the data, if only by a little.

I would suggest right-sizing your columns from the get-go. You need to take into account expected growth of the data versus the life of the application. If it will take 100 years for your integer data to require an INT datatype, then you can likely stick with a SMALLINT if you think your application likely won't live that long. If it will take only a few years to get there, then it would be easier to just handle that right now rather than risk an application failure when the upper limit of SMALLINT is reached or to risk forgetting to fix it when it gets close.

If you don't, your application will likely fail at some point, likely in the middle of the night or when you're least expecting it! Or will fail for someone else who is tasked with maintaining your application.

If the users can live with having your application be down while you determine that your data type has exceeded it's size and take steps to fix it, then you can wait. I would bet that is not an acceptable scenario though and you would like to keep the application up time as close to 100% as possible.

Don't wait for "someday" to arrive, get it right today!