Can you buy a crap laptop but install a graphic card to make it good?
Yes and I'll tell you exactly how to do it.
Acer is the best brand to do it cheaply.
You need to find a broken, budget machine. You can get one
around 50$. If you do research by looking at pictures of
motherboards, you can tell which are based on the same case and can
use much better motherboards with standard MXM 2.1 type II
graphics. The ODM model is useful to know, because it applies to
the entire case and every model it is based off of, but on Ebay
they aren't often listed like this.
I did it with an Acer 5315. I bought a 5720g motherboard and it
can run a 9600m GT ddr3. I had to cut holes in the case for some
ports and buy the graphics heatsink. The ODM model here is ICL50,
and there are in fact more which use the same case such as ICW50,
the AMD version, ICL70 is the 17" case for the same motherboard
layout.
You really have to know what you're doing needless to say.
Its in fact easier for high end models, and you have more
options for cards and less work to do, but then you are investing a
lot more money. the Alienware m15x can be found with i3 CPU, Nvidia
gt 240m and upgraded to AMD 6990m. The first step however after
being a very good technician is really knowing the market and what
is worth it.
Its certainly more interesting but much more difficult to mess
with other rare brands and models like Acers. There are a few out
there like the 8735g which use the same interface as the AMD 6970m
etc and could potentially be upgraded a lot. Theres also a few
gateways, like the nv73, nv74, nv78 series, but these are not
necessarily upgradeable unless you know that they have a dedicated
card already.
Clevo/ Sager laptops are meant to be upgraded, but you'll still
have to pay a lot for one. Do research on anything before
buying.
The best laptops around to upgrade are the MSI gx640, gt640 etc
if you can find one for cheap.
But you said crap laptop. Yes the only way is to find broken
Acer machines, cheap motherboards etc. At this point you are
basically going into the repairing trade and might as well learn
board level repair.