My suggestion is if you are just curious and want to see if you'd like the game; ask the group of people playing if you could play one of them in a small skirmish using some of their models to test the waters out.
Before you get an army, get a copy of the rulebook and read it. Then browse the Codexes(Army books) and see what army fits your play style, or ask around and someone could suggest one. I play Space Marines and Necrons. Space Marines are fairly noob friendly, but shine when you really get to know the army. Necrons are fun, but hard to play right, I also haven't played them with the most recent codex.
For getting an army up and running quickly, check out ebay. It might be hard to find a complete army for 100-150 bucks. Starting from the ground up is fun as you can really tailor your army your way, and if you like the hobby you will acquire, well lets just say "enough :D" stuff to combat any foe. Price difference really comes down to whats out there, i've seen whole armies sell cheap but that's kind of a rarity.
Metal v Plastic: Plastic all the way, pretty much all you can find now-a-days aside from second hand sources. Plastic is easier to cut from sprues, clean, and assemble. Plastic is also more durable, in my experience, to drops off the table and normal wear and tear. If you are new to hobby modeling in general, stay away from resin at first, its hard to do right plus its highly toxic to inhale the dust.
As for forming coalition armies; in a word yes you can but there are rules to it. In 7th edition you can take a primary detachment and then bring an allied force. I would suggest sticking to a single army at first. But generally speaking, not all the models need to be painted the same color, just from the same Codex. I have a friend who plays Space Marines as well and has his marines painted from all manor of chapters(organized bands of battle brothers).
Quality from Games Workshop is really top notch, they have come a long way with their quality and their price reflects that. Breakage depends of the model and how it's treated. Infantry models are really durable as are most vehicles, just need to be careful with the thin bits.
A good starting point total i think is 1000 points, it allows you to field cool stuff, but still really think on how to build your army from your available models
Basic assembly of most of the Games Workshop range is fairly straight-forward, if you've built a model tank or airplane you can do this. Painting is as easy or as hard as you want to make it.
I know this is a lot of text but, my last thing is check out forums specifically for Warhammer 40k like bell of lost souls or dakka dakka both have great information. Also don't be afraid to ask any questions, we are here to help :)