WH40k: A Query

So I've been poking around tabletops for a while, and I've heard good (and venomous) things about Warhammer. I did some research on my own, and have come to the conclusion that HOLY CRAP is it expensive. But... It's still interesting. Most of the rules I can figure out on my own if I ever scrape together enough to buy a codex, but for now there's a question niggling the back of my mind... Could one play a Noise Marine/Rubric Marine/Tzeentchian Firelord army? Or would that simply be unfeasible, or not allowed in the first place?

Since i do not own the 6th edition Codex for Chaos Space Marines I would not be able to answer your question with the most updated knowledge over the subject, but for 5th edition rules the answer would be a yes. You can use multiple warbands in your Chaos Space Marine army.

Generally army lists arn't that restrictive.  You shouldn't have too much problem building any list you really want so long as you meet the points/troop requirement.  I myself run necrons and tend to play very heavy amounts of footmen robots.  So while they tend to fall over more often than my opponents tanks; there are plenty of them... And you know necrons, We'll Be Back (Special rule to stand up after dying).  HOWEVER if money is an issue you can always look to warmachine/hoards.  That game is just as fun as warhammer 40k/fantasy with a cool steampunk with big warjacks (robots) in normal warmachine while hoards tends to be more primal with large beasties.  But the game costs a lot less and you can buy paint sets made for different armies if you have a hard time trying to figure out how to paint things.

Well that about answers my query, thank ye both kindly. If I do get the money, I will have such a squadron to annihilate the hordes!

One thing to keep in mind is that there are two ways to play 40K. The first of most common way is to dot all your I and cross all your T's, to buy the figurines and paint them one of the color schemes form the codex, this way is easier, takes less time and effort. The other way, the FUN way is much more open. With 40K you can model and paint your army is a variety of ways. For example I started making a Tyranid army based off the flood from Halo. My normal little guys are zombie like humans, my big monsters that spawn more little guys and bio-organic incrusted tanks, and my leaders and huge monsters/demons. Just remember that to be in a tourny they have to be 50% games-workshop pieces/parts. I'd say the general rules for going this way are two fold: 1) goes it fit within the game universe/machinics? 2) Does it look cool.

Whoah... that's an option, like, in tournaments? I mean, I knew you could do that, it made sense, but that they allow it in the leagues and stuff is pretty cool. I might just do something like that... come up with my own fluff and whatnot. That could be fun...