This thread has multiple questions:
1.) Build or Buy - Is it better for me to build my first AR from parts (all bushmaster or similar quality) or buy a bushmaster from a place like AIM Surplus? This will be my first AR but I am fairly good with firearms in general; SKS's, Nagants, Makarovs, Benelli, and Beretta shotguns.

2.) Parts - I understand that diffrent manufacturers make diffrent quality products but what are the components of the AR that I can compromise on to save money. The only major thing I can think of is that I want crome lined barrels but i dont care who makes them.

3.) Lowers - For my seccond AR I am going to be building a CAR-15 clone (not SBR though) and I want a lower that is not as much $$ as a bushmaster. What is the diffrence between a mega lower and a bushmaster. Also what lower priced lowers do yall like?

Slow down take your time and look at reviews of parts and building procedures here on ARFCOM. There is more than enough info here online, but you could also get the AGI AR-15 armorers video and a book by Duncan Long called "Build Your Own AR". Both of these ar available through Brownells link Read online about a few tools that you will need for building your rifle/carbine. Here is a link to Build Your Own. Read over this and see if your up to building one, its real easy and cheap. But you wont be saving any money.

The best advice is to use searches here in ARFCOM, big help brother!

RRA lowers are very nice in Quality and Finish, not to mention that they are affordable. There is also many other affordable lowers (Exmp. Stag arms, Mega, and More)

build man, model 1 sales kit and an ameetec lower, you are good to go

1.) Build or Buy -
Buying a complete one isn't a bad idea, but you could do the best of both worlds... get your upper and lower separately and save some money. Have your upper assembled for free if you get all the parts for it from the same place. I'm having eagle firearms build a couple of uppers for me, a 20" Bushmaster HBAR on a Rock River Arms A2 upper receiver, and a 14.5" Bushmaster M4 1/9 Barrel on a Rock River Arms A4 upper receiver. Both with RRA parts. I'm saving more than $100 each upper, over buying from Bushmaster.(and little wait time).

2.) Parts -
The Bushmaster barrels I mentioned above are chrome lined (that's why I got them, rather than RRA barrels)

3.) Lowers -
A complete MEGA lower with your choice of parts kit (Bushmaster, RRA, or Olympic) and RRA 6-pos M4 stock cost $230 from Iblis/Carricoz ARmory. ar15.com/forums/topic.html?b=7&f=21&t=153544

Going through this process rather than buying a complete rifle can save you at least a couple hundred bucks, and all you've got to do is push two pins in when you mate the upper with the lower.

+1 for build.  All the technical resources you need are right here.  You may have to buy some tools, but any good AR owner should have some of them anyway.