If you live around Nashville, TN there is a brand new Sawstop 3hp PCS with 36" T-glide fence for sale for $2027 on ebay. You can make an offer less than that though. This store is where I bought my PM2000 for less than $1500.
Just use plywood with edge banding on the sides of each door. It's dimensionally stable, it's tough, and it's nice looking...especially compared to hardboard. Route a shallow groove in the bottom shelf and a slightly deeper groove in the upper shelf so that you can remove the doors if needed.
One word of warning...if you plan to use an upper and lower track system of ANY kind, you'll need to make sure that both tracks are perfectly square and in line with each other, otherwise you'll get all kinds of binding.
It robs you of some overhead clearance but, for doors that large I would hang from above like sliding closet doors. I have a space like this in my shop and it has worked out very well. The doors are a frame as you describe but, skinned on front and rear (1/8") to add rigidity and reliability (resist warping).
I never go into "design mode" without them knowing what the numbers are going to look like. I just don't care to waste my time designing something I'm not going to build. They either are ok spending that kind of money or they aren't.