Read, look at, analyze (the more, the better!)
Simple pieces of advice, which are useful to follow.
You should buy what you like. There is no universal recipe for a prediction of the potential growth of a model. One can only try to predict trends.
The more you will base your opinion upon your knowledge and experience, the faster you will develop your intuition about the watch market. Systematic research of specific online-platforms and forums will not only give you an understanding of the market. Still, it will also allow you to single out the liquidity of certain brands at secondary resale.