E-300 AMD... not all of them are but a lot are soldered directly onto the motherboard. Is it impossible to replace it? No, nothing is impossible... but yes for all intensive purposes it's impossible.
GPU... same story. It's possible to hook into a laptop's PCIe in one way or another... but the price of doing something like this not to mention time... you do it because you love doing projects like this, not because it makes sense.
For low cost gaming... Lenovo has pretty decent prices for Y40, Y50, Y510(older) series laptops. Their build quality isn't premium but to get to the price points they sell at they had to cut corners somewhere.
A desktop assuming you already have supplies like keyboard, mouse, monitor... would probably be the cheapest way to get a "gaming" grade computer. Some people will even say "there is no such thing as a gaming laptop".. to a certain extent they have a point, there is a reason a desktop GPU is about as big and weighs more than most laptops lol.
It really depends on your budget. I'd say get the best you can for what you can afford and get it at a good price by watching deal websites like slickdeals.
I don't really enjoy giving advice like this to people who aren't familiar with computers and their components... because I need to paint with a such a broad brush.. I can't really say "here buy this exact one, it's perfect for everything" because it just doesn't exist. Depends so much on budget, what you intend to use it for, how long you want it to last, so on and so forth.
This site is great for ideas... but I would never say "yeah buy everything from this line". It's for ideas. It's for Desktops
http://www.logicalincrements.com/
These websites help you compare mobile GPUs:
http://www.notebookcheck.net/Mobile-Graphics-Cards-Benchmark-List.844.0.html
http://www.notebookcheck.net/Computer-Games-on-Laptop-Graphic-Cards.13849.0.html
This can be helpful when comparing desktop and mobile together... but far from "all inclusive"
http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
This site I feel is a little bias if only for the benchmarks they use... but I still use it and like it:
