Okay, I'll take a stab at it.
Different Types of Habitats
So first off you mention an essential habitat for their survival, so I'll start there. There are two possible types:
If this habitat is artificially constructed, you're going to need some engineers and maybe an architect or two. This is so the structure that's built can be maintained. Inside, assuming it's designed to be insulated from climate change and environmental degradation, you are going to need a ventilation system, and if there are air-born hazards an air filtration system as well. This adds HVAC Engineers and HEPA Engineers to the list.
Considering power-generation needs, if you have the resources and expertise nuclear is the way to go, as it provides the most power... otherwise, you'll have to go with renewables like solar, wind, and water. I'm going to assume nuclear is out of the question and target the renewables.
- You'll need Solar Engineers for this.
- These panels would also need to be cleaned off regularly in sandy or snowy weather conditions.
- These wind turbines would need Wind Turbine Technicians (also known as windtechs) for installation and maintenance.
- These are best if your Habitat is located in a plains like area clear of obstructions for the wind.
- You'd need a flowing source of water and some hydro-electricians for this.
- If you were to go for this make sure the habitat is away from the damn, as the water build-up would eventually form a lake (may be to your advantage if you habitat was on an island in the middle of said lake.)
You mentioned a lack of food and water so assuming the damn helps with water you can get the most bang for your buck with hydroponics for your food! This will probably need some Botanists to really help maintain the health of your crops.
You have to make sure your people don't get sick and spread disease, so go ahead and add Doctors to your list, and remember there are many different types of doctors, so you'll have to take that into consideration as well.
You'll need guns, lots and lots of guns. A Mechanical Engineer would probably be best for maintaining the guns and somebody ex-military to teach others how not to shoot themselves in the foot.
This is for low-budget habitats that are built by-the-people, for-the-people.
For low budget habitats, you don't actually need anything fancy, if you or your people find dilapidated housing or even an Abandoned Base they'll be pretty well off, basic repairs are a learn-as-you-go type of thing.
This would be trickier, assuming they didn't have access to generators and such they'd need at least an Electrician there are some interesting tricks you can do with a high voltage DC motor to generate power: DC Motor Wind Turbine
Otherwise, you'd have to have an expedition to the nearest substation or power plant.
Any nearby survival/outdoor sporting goods store (assuming it hasn't been already ransacked) will have Iodine, drinking canteens and self-filtering water bottles.
This gets tricky... as most people will automatically go for grocery stores for canned goods etc. remember the outdoor sporting goods store? go ahead and get yourself a hunting bow or two
Again, the outdoor sporting goods store should have some guns and ammo as well.
SPORTING GOODS STORE!!! it'll have first aid kits, or if your lucky most firemen, police officers and of course EMS have at least a little medical training.
I think that's it... Hope this helped!