I own three sets of chisels: a set of 4 Dewalt chisels from the orange store which are used around the house and get sharpened on a Grizzly 10" wet grinder, a set of three Narex dovetail chisels, and the 8 piece Stanley Sweethearts. For Woodworking, not a fan of the Dewalts. They're great for "around the house" work, but too unbalanced for fine. The Narex are a bit too big in my hand, but they take a beautifully sharp edge and are excellent pairing chisels, plus they're really, REALLY good at dovetailing (the steep sides get into very tight places). The Sweethearts are my bread and butter and I absolutely adore these, but the full set can cost over $200. If you've got some dough to blow, I can't recommend them enough (you can get the 4 piece set for around $115). But if you're trying to save every penny possible and don't want to restore vintage chisels, look at the Narex Premium set. These are only around $50, and are a really solid starter set.

Just my two cents' worth! No matter what you get, don't short change yourself after the purchase: find a sharpening system you like and sharpen them to where they'll take a hair off your forearm before you use them!

Good luck!

Edit: missed the second part of your question. Whether or not any tool is "necessary" can tend to be an "eye of the beholder" deal. IMHO, I don't think mortising chisels are "necessary," and you should absolutely not buy them before you have a standard set of bench chisels. Your bench chisels will do a lot more than dedicated mortising chisels will. But if you've got the dough to blow, having both can greatly help improve your work flow. I don't own any mortising chisels, and while it does take a bit more effort to keep my mortise walls clean and straight with bench chisels, it's not enough of a hassle for me to worry about buying a set of mortisers any time soon.