We are, once again, getting into the semantics of HBU and how far it must be developed within the context of an appraisal. The question being asked isn't what exact structure (size, quality, condition, color, etc) would maximize the value on this site, but rather who is the typical buyer for this property at this time and, in fairly generic terms, how would they use it.
Obviously, the Highest and Best Use for this site would not be to leave it vacant, but to develop it to whatever the HBU is. By the description being given, the typical buyers for this type of site are buying them as redevelopable sites rather than their current use or as another use. And we know this because those sales with similar development potential are being redeveloped whereas those sales without such potential are remaining in their current use.
I'd do it as land, on the land form , but include a substantial HBU analysis that goes through the criteria, so that the reader can see you didn't pull your conclusion out of thin air. The sales will include not only the most direct comparables (units being redeveloped), but also some other sales of vacant or underdeveloped sites with similar zoning. If your conclusion is correct, all of these data as a whole will demonstrate the trend.
Don't forget that if the trend for redevelopment of properties with similar development potential is going to be the primary issue in your valuation, it should also be the primary theme of your report. Start discussing the trend in the neighborhood analysis and continue developing the theme in the site analysis. That way, your readers won't be surprised when you get to the Sales Comparison and you're valuing it as a site.