TraderBob,
I have built and repaired more PC's that I can count. If you are a serious trader, you need a fast and reliable computer. Since you are not a techie, and someone else could use your existing computer, than it makes sense to buy a new unit. Here's some things to consider.
Many manufacturers use non-standard components. Compaq, HP, Sony, etc. This can be an issue when trying to upgrade in the future. Make sure the case is an industry standard ATX. Unless space is an issue a mid size tower is a good size. Baby ATX frequently don't have room for extra drives, DVD and CD R/W.
Also many manufacturers have their own hardware drivers that do not come with the standard windows installation. This can be a pain if you upgrade from one OS to another and you do not have the manufacturer's disk. You have to download all the drivers. Make sure the manufacturer uses mainstream components and this won't be an isssue. Dell is pretty good all around. In computers, as in most things in life, you get what you pay for. Your very cheap ebay type machines are usually limited in one or more way - fewer expansion slots, lower quality video card, slower hard drive, less memory. If this is what you are looking for that's fine - but buyer beware.
Mike