What numerical aperture and image circle are we talking about? 0.15 NA leaves a lot on the table (at least 0.6 NA objectives can be had for still-decent prices).
But I'd probably look at a Zeiss Planar style design.
In general, a 3/5-element design (achromat/apochromat) should do the trick I think, provided you have access to arbitrary aspherics. And in case you can accept a shift in focal length with wavelength (like monochromatic photolithography would), a single symmetric element should be up to the task, unless I'm missing something.
But I'd probably look at a Zeiss Planar style design.
In general, a 3/5-element design (achromat/apochromat) should do the trick I think, provided you have access to arbitrary aspherics. And in case you can accept a shift in focal length with wavelength (like monochromatic photolithography would), a single symmetric element should be up to the task, unless I'm missing something.