Nice design!
I agree with the general consensus that the corner pieces will be expensive due to their size, I would estimate in the region of £25 - £35 per corner piece when using a company such as Shapeways.
You shouldn't however be worried about strength, the SLS process that Shapeways use results in parts that are pretty much on par with injection moulded plastic parts in all planes.
Accuracy wise, I think you can get away with tolerances as low as +/-0.3mm over the length of your parts (around 200mm?) for hole to hole distances, plus I would add a further clearance of 0.1mm - 0.2mm between mating parts/surfaces/edges. I have found the SLS process to be very accurate, especially if you follow the guidelines for obtaining quality parts to avoid distortion.
Surface finish wise, I agree that some 3D printed methods can result in bad surface finishes such as FDM.
But, some of the latest machines can print really fine detail with almost polished surface finish, an example of which being the Objet machines that you pointed out, however most of their 'plastic' simulant materials are quite brittle.
SLS is the best all round process that I am currently aware of, the parts are strong, accurate and can be finished up to a near smooth finish for a small cost. Even in their unfinished state they look good with a fine matte surface texture, this can actually work in favor of your aesthetics.
