If an eyepiece is used, a simple orthoscopic type is better than a complex and costly wide field type. The desirable lenses mentioned above are usually designed specifically for image magnification ratios of about 10 to 1. They are all of a speed of f4 or faster and thus will easily accept the object light bundle without vignetting. Edmund Scientific is a source for high quality lenses of this type. There are other sources such as surplus optical suppliers.
Some photo shops have selections of used lenses that are also suitable.
You may have to have an adapter tube or some sort of holder ring made to mount them in the tubes you are using. Typically the microscope objectives are designed to work at a projection distance of 160 to 250 mm. So a 10X objective will be just about right for many purposes. Such an objective has a focal length of 16 mm. They are very excellent for this purpose.