No you don't need PS Plus to buy games, just a PSN account. You can redownload and play your games as well with PS Plus since the games are tied to your PSN account.
PS Plus is purely optional and doesn't effect you if you don't sub to it. It just gives you free games that you keep for as long as you are a PS Plus member, betas, one hour time trials for quite a few games, and larger discounts on things on sale and when you purchase something on sale with the PS Plus discount it is yours even when the membership to PS Plus expires.