Elder beads are really a monk "must have" item and something to hammer off of and heal up with during downtime for Necros, nothing too crazy. Just buy a couple
Stalking Probes for those 'ohshit' moments when you need to tap but don't have a target. Buy 2 or 3 so you can recharge them, the cost is something like 1plat per recharge versus the 120k+ beads go for now (that's nearly 600k uses of stalking probe...).
CoS on the other hand is a game changer, in that it literally allows you to play the game differently. CoS allows you to spend 100% of the time you want to be invis, invis. Throw in FD and Lich faction changes and Necros are able to get pretty much anywhere they want to be. Don't listen to people to try to marginalize this, but also don't pay more than 10k for one. Root rot in the middle of zones without fear of wanderers, stand up immediately after a FD, run around in all the goodly cities that you want, etc... tons of uses for a CoS that every class would kill for.
There's not a ton of great charm spots for Necro (that I found) until 50+, before then there's usually better Exp+cash spots. Charming is my favorite way to level, so if you have luck somewhere more power to you. Before CoM or HS, I had some luck in Burning Woods at the undead ruins (2 on map near Chardok), but had a better balance of Exp+cash in Wakening Lands. One of the best things about Necro is we can fear kite, charm kite, root rot.. tons of different ways to level so you never get bored. Necro is probably the best implemented 'jack of all trades' in Classic... which is why they saw so few changes when compared to other classes in Kunark and Velious.