A fixed 10x is a great choice for staring and learning how to use the Mil Dot reticle, and other ranging reticles, for their intended use. You will get the basics down and not break the bank.. Bushnell has a 10x40mm that would be the ideal choice for that purpose.
If you plan on doing a little more on the target side and want to dial in for more precise and accurtate shots, more magnification could be useful. Its best to step up into a nice quality optic for that. Great choice would be the Vortex PST series. The 4-16x is definitely enough magnification but they do have their 6-24x as well. Also nice if you plan to shoot further in the future. Ther reticle's are matched to the turret, MIL/MIL or MOA/MOA. A great feature when target shooting and ranging. Also have the option for a First focalplane model which allows you to range at any magnification. Pst has excellent glass and a solid construction and would be a great choice for your purpose as well: