Basically any of the starter campaigns will work . Things like forgotten realms are easy to get into and will work with basic characters. They are primarily combat based so the DM doesnt have to worry to much with RPing the group from place to place and dealing with the inevitable attempt to break the campaign. This should prob be your first buy, or something like it.
If you want to go outside of just the basic books, Players hand book (PHB)2 and 3 are good places to start. They will vastly expand the races and classes available for more diverse group interactions.
The various power books are optional to further expand classes but not necessary in any function to play the game. Martial power 1 might be nice just so fighters are less linear in how you build them.
One of the adventures vaults might also be good as the original PHB isnt exactly overwhelming item wise, though PHB's 2 and 3 will also expand your item list.
Once your DM gets more confident you might want to try creating a small world based on D&D back story. Same gods and such but your own map, settings & history. Its an easy way to help your DM get more into it without having to memorise innumerate random little things to make sure its done right. If something comes up you didnt think of. Make it up. Its your world.
After running a forgotten realms your DM should be well versed in running combat. Its only a small step after that to creating your own combats. Just remember, there is a REASON DMs use screens. If your combat is being walked over, pull in a few more mobs, invent them a new ability, and/or increase health. Fluff some rolls so you hit more attack (ask them what there defence is NOT if 15 beats it, this way what you rolled cant be irrelevant if necessary).
Being a good DM is the hardest part of D&D. The reason a lot of DM's do combat based campaigns is because they are easier. You can only "break" a combat so much. In RP you might need your characters to go here, even if they dont want too. Your job is then to steer them that way WITHOUT breaking the 4th wall (wiki it). If you have to resort to saying "you cant do that you have to go here" your not doing a very good job. Its a lot harder than it sounds especially since many player will go out of their way to push the limits of what they can do. Learning to "go with the flow" will be key, and I've known VERY few DM's who can do it properly.
A few hints. If a player wants to do something like, Idk, run and jump off a balcony, grab a chandelier swing 90 degrees land on a chair back with falls over depositing them neatly on their feet in front of X person (bad guy or otherwise). Realistically you will need scores of 50 odd on both athletics and acrobatics. Dont make em roll this. If its awesome, and character appropriate, page 42 it. Get em to roll a D20 and rather than add the relevant score, define that they do on how well they roll, if they get a 20 describe what they do with massive flourishes and embellishments with probably some bonuses to whatever they were going to do after that, if they get a 1, they run at the balcony, jump and catch their foot on the rail, sending them headlong into the chandelier, after doing a few rotations by the knee they fly off, bounce off a wall and land on a table destroying it, place them where you want. Taking Y damage. Either randomly assign damage or have then roll a dice. Doesn't matter.
If you build a campaign on this idea you can get some BRILLIANT results. Such as people he-manning though a solid wall while 3 other party member argue, and another realises for the first time there is a glowing yellow thing in the sky, and it appears to be watching him.
Seems simple enough but you'd be amazed how many people stick to the rules through thick or thin. At the end of the day its your world. Do what ever the hell is awesome.
There was something else I was guna put but I forgot. Ill come back when I remember.
EDIT: Dont worry about sticking to monster manual monsters. Especially at high levels it can be a pain in the ass to remember the array of abilities they have and what is ready if you are using lots. Make up your own with only 2 or 3. Makes it so much easier. Give artillery something like, 1 strong long ranger attack on recharge, one basic and something like reactive teleport when hit so they have some survivability.
EDIT2: Dont forget to look up the ability erratas. They fix things that are somewhat overlooked on release. For example fighters have an at will called footwork lure. In the basic book it means if you have someone with reach 5 (which you can get), you can attack someone then move them adjacent to you. The erratas modifies this to "move one square closer to you".
