You mention that you'll be riding 20 miles r/t and that 80-90% of that is "trails/tracks off road". What kind of shape are these trails in? Are they like paved trails that motorized vehicles are not allowed to use or are these rock and root strewn cow paths? If it's an improved and smoother surface, any of the bikes should be up to the task. But if it's really rough, you may want to conside a 'Hardtail' mtb.
Also, what size frame are you looking for. I started to poke around the Evans site, but looks like they essentially act as a clearinghouse of sorts- NOS, limited size selection.