Collect what you like, make sure you have the essentials (sharpener, oils, basic solvents, and a place to keep them) and if you are going to store a knife for any length of time I'd clean it very well and apply a light coat of mineral oil. If you have any brass, silver, or copper components on you knives make sure that they are clean before storing them, and if you have anything tarnished (that you don't want to be) get some quality metal polish. On a related note leave any patina on older pieces as is, don't try to clean it up if anything it will decrease the value of the knife. If you are collecting knives to use you'd be set with a good sharpener and some oil (rem' oil, mineral oil, whatever) I use a lanskey sharpening system that works well for me but there are a lot of more modern options out there that are easier to use and just as effective if not more.