The designs and material choices are so good that it doesn't really matter that they are inifinite. If you do a lot of work at oil make sure that the fine focus is smooth and progressive before you buy. There are two main types of microscope frames, upright and inverted. The upright is the most popular type while the inverted is used in metallurgy, cell culture and aquatic studies. If the specimen you are using is in a suspension or very large an inverted may be the microscope for you.
Upright microscopes come in three frame sizes student, clinical or bench top and research.
The student microscope is the smallest and the least versatile. It is designed for bright field and possibly phase. The design goal for a student microscope is to be inexpensive and tough to break. Usually these microscopes will not have Koehler illumination but use diffusion illumination. They will not have the light output of a Koehler illuminated bench top microscope.