Necromancer is kinda annoying because of the need to Soul Siphon everyone early on. Monk is a good one for the piercing and deletion, if you can beef up enough with health and healing. As for talents, Heal is a good one to start, as is Portent for mages so you know what spells to stick with. At the end of the first floor, get an equipment slot most of the time unless you already have at least one or two. Especially important if the second floor boss is the Hydra (which you will know if you have Portent, of course). If you have a slot and the Cloak of Flame or whatever it is called that does two fire damage for every attacking card the enemy plays, the Hydra will literally kill itself.
If available, get a Deck of Wonder. One of my all-time favorite equipments, right up there with the Cloak of Flame and the item that protects you from all damage on your turn. Deck of Wonder gives a random card every turn, allowing you access to some powerful stuff. Also good for dumping if necessary.
Polymorph is good, especially when you do make it to the third floor. There are some truly horrible enemies down there, like the Hand of Glory. Being able to switch them up is helpful.
Also, this for much later, but once you save up the points for it I always recommend buying Master Thief. It is the most expensive to unlock at like 20,000 points. It is also worth it. Make sure you buy nothing on the third floor, and if the stuff in the stores is good enough, you take it at level 10. Clear out everything of use, then hopefully there is a lemonade stand, church or anvil to burn your stored gold you will no longer need.
Another thing: Preparation is a beginning talent option, and is highly recommended for classes with cool abilities that activate outside of battle, like Professor and Dragon. Lets you use them almost twice as often, and makes already powerful classes into juggernauts. I have taken both of them all the way through to the end in part because of Preparation; Professor was my first Lord of the Dream killer (copy a bunch of that Discharge-like spell the wisp uses, add in some Three Wishes for flavor, and don’t forget multiple of those bleed stackers from the Piranha), and Dragon was my third (Shrink, that awesome spell that heals you up to 40 and does that much piercing damage, and some Coup de Grace fun with loads of actions from chomping on Thieves and the like, plus an extra card from a lucky Undead snack). This game is my Dark Souls; it is cruel to me, and I love it for that because it makes the scant few successes all the sweeter.