Sorry, but I think you've been bypassed by technology.
At one time there were boxes that did this, I think Roland did at least one 'midi file player'. The idea was in effect a specialised computer (in effect) and a limited sound module unified into a single box. But I think this would no longer be economically viable. The price of a laptop has come down, even quite fancy ones can be had second hand for 100 or so (dollars or whatever just about). And the sound module bit, and the expectations as to quality from such, have become more sophisticated, and varied, and specialised (as you yourself are indicating).
Options have arisen though regarding the Virtual Synth, with soundfonts, that might get you back towards 'possible'? May be the only way to get what you want. You'd need a new-ish laptop, depending on price, with some basic software that could use VS technology so that you could find and load specialist soundfonts. I've been using the SynthFont package which is in effect a simple midi file playing package with very limited editing, but which can access a sound font package which could be built around whichever specific instruments you want/need. Maybe the sound quality you want would mean you needed to buy some fonts, but you could get going with free ones. I've been using something called 'Timbres of Heaven' and it's not too bad. At least this route would keep you to one box. A different player package might need an external sound module, in which case you'll need to find one box that has all the sounds you need.
Might this be any help?
Geoff