I had an ST120 for many years. It's a good basic scope for widefield views, and the eyepieces it came with, and the diagonal ( a 90 degree one, not 45 degree erect image one which you will need for terrestrial views) were fine....not the best by any means, but fine for starting out. Trying to get decent results at higher magnifications than you can get with the supplied 10mm eyepiece is a waste of time. It's a basic achromatic short focal length lens, and these sufferfrom chromatic abberation and can only take so much magnification before the image becomes a blurry mess.
Don't get a barlow with it.....the last thing you want is more magnification. Used for what it's designed for, widefield views of star clusters etc, it's a great scope for the price, but if you want high magnification views of the moon and planets, then get a Mak or similar, as has been suggested here earlier.