The good news is that teams were building great CAD models five and ten years ago, using the technology that existed at the time. That should confirm that you don't need the latest, greatest processor and video card to do meaningful CAD.
At home I use a five year old desktop computer running Windows 8.1, with an Intel Core 2 Quad Q8300 processor running at 2.5GHz. It has 4GB of RAM, and an ATI Radeon HD3850 video card. For most basic parts and assemblies, it runs Autodesk Inventor 2015 just as well at it did five years ago.
It does bog down a bit on bigger assemblies... I downloaded the "Simbot Evolution" STEP file from the CAD section of Simbotics website (thanks, 1114, your CAD files make a nice benchmark!). It takes quite a while (maybe a minute? a bit more?) to open the file. It moves a bit jerkily when I zoom, but actually rotates okay. Edits can be... well, tedious would be a good word... but they do happen. Eventually.
To be honest, I wouldn't want to use my computer to edit the entire assembly of Simbot Evolution. I could probably speed the process up quite a bit by turning off parts of the model that I don't need to see. Or by realistically asking if I'm ever going to need to design something with as many speed holes as that beast! But the fact of the matter is... it does work, and on less detailed models it works fabulously.
The Passmark rating for my CPU is about 3000. Compare that to the CPUs on this chart and you might have an idea of a minimum CPU performance level
https://www.cpubenchmark.net/laptop.html
The Passmark rating for my video card is about 400. Looking at this chart will suggest you can't buy a discrete video card that slow any more
http://www.videocardbenchmark.net/mid_range_gpus.html
And get at least 8GB, just because you can.
In the fall I had the opportunity to review a Lenovo P300 (
http://hbridge.ca/wp/?p=53)... an entry level desktop workstation. I could drop 12 copies of Simbot Evolution into Inventor before the P300 started to bog down. That was nice, but probably overkill for your needs.
A $300 laptop should work great for modelling parts and simple assemblies, but if you want to crunch a highly detailed full-blown Simbot style assembly, you will definitely enjoy the luxury of a more powerful machine. Depending on your modelling needs a $300 laptop might make a great CAD machine... I know the only time this old desktop of mine really bogs down is when I do silly things like load Simbot models or do a lot of RAW photo editing.
Jason