Cockrings are generaly good to be used with a pump as they provide a better barrier to avoid your balls being pulled inward (which hurts).
Also, manual pumps are more accurate, but if you are lazy automatic is just more convenient. But idealy you then do want to have one you can adjust. Some pumps only have a single power level which in most cases is just too strong to start with.
If a pump allows you to set the levels like for example 25-60 (i think its mPa that they use as unit), that range is usualy good. 25 is a decent enough suction, and 60 is a very high one, often close to unconfortable levels. Thats the sort of automatic pump that can be worth it.
For arousal levels pumping is usualy not very effective on its own. It only causes your nerves to become more sensitive, so anything after it can be a lot better.
But make sure to not overuse it. Too regularly, too long and too strong suction can all cause permanent damage. For example it can make it harder for you to get your dick hard (your body can start to expect suction to aid this, which weakens that part. too long and too strong can cause tissue damage).
And while dick size increasing does happen. Dont expect wonders. Its only a few percent that it will help. So a 14cm dick might become 14.5 and with some luck 15. Repeated use can keep the effect a bit longer, but once you stop its back to normal in just a few days.
(and realy… most women dont care here! most of their sensitive locations are only a few cm inward, which most dicks will do a fine job at)
You realy should only use a pump if you enjoy it yourself. And occasional use can definitely alter some sensations you are otherwise already used to. And sure, that single time you use it just before penetrating a women, it will also change the feeling for her.
And sure, it might look larger, but most of the time what you actualy see is that because the skin is forced to stretch a bit, it simply looks harder than usual. Which can be intimidating (and fun) on its own.
Manual pumps however are better when experimenting, they are cheaper and safer.