A Good Microscope
A good microscope needs a sub-stage condenser. Without one, the high power objectives cannot do their job. The sub-stage condenser will also allow the use of special contrast enhancing techniques and devices, such as dark field illumination, oblique illumination, and Rheinberg illumination.
When you decide to upgrade your microscope, a microscope with a modest sub-stage condenser can be fitted with a better one. A microscope without a sub-stage condenser does not have a place to put one.
DIN standard objectives are another thing a good microscope will have. This allows you to replace the inexpensive ones that came with your microscope with better ones that also conform to the DIN standard. You can buy the less expensive microscope and get a lot done with it, and upgrade it later as the interest grows, or your finances grow, or as the nature of the work you are doing requires.
The objectives are the most important part, followed by the condenser. That is where you should put your money.