First of all, don’t buy based on specs, buy based only on your experience. If you don’t have first-hand 3d printer experience, get some any way you can, through a library fablab, a CNC build club, a meetup, a hackerspace or makerspace, or anyone you can find through 3dhubs, for instance.
Second, scale and level of detail don’t have anything to do with each other, and the quality of good small details varies a lot based on many variables, but … you want to print things at HO scale which suggests you’re doing trains or small model trucks, dioramas, things like that. I don’t think you’re going to get the kind of detail that will make you happy for very small things like a human figure at that scale, but you might get a boxcar to come out ok, or a brick cottage, things like that. Check out resin-based printers, there are some that are quite tiny (under 2" build area, some of them) but are inexpensive and produce cleaner models with more detail. The liquid used in them (resin) costs about $80 a liter/quart, though. Order some test prints from printers you want to consider as a start, and see how they turn out.
Third, the “minimum microns” probably refers to minimum layer height, the easiest axis to get the illusion of microdetail. It mostly means that there is a minimum elevation change between layers, otherwise the print head can’t lay down another bead of melted plastic on previous layers. Smaller layer height means longer print times for a given size print, keep in mind.
Lulzbot is great, and the Lulzbot mini might be perfect for you … but only if the quality of prints it generates is going to make you happy.
Try different materials, and consider trying to get some made with sculptable filament http://3dprintnews.co/2016/01/sculptable-3d-printing-filament/ which can be smoothed and stippled, textured, carved, after printing. It can be a game-changer for organic-looking prints.
