So last time we looked into this it came down to the price of all the parts, mirrors, mounts, tube, power supply, head. The least expensive way to get into this is buying a full unit from ebay, at that point just throw in a ramps board and call it a day. That is why I stopped looking at them. I even tried to get a group buy from one of my vendors that sells them, said we could get a slight discount but still cheaper to get a full unit from them than just the parts.
If you want to try it I would love to see it! I would mount the tube up one of the table legs, bounce it 90 degrees off a corner block, 90 from a roller assembly, possibly straight into the laser head, depending on geometry ( it think they have a 90 built in), if not like you said 90 off the gantry into the head. The bad part is you need to be able to steer/aim the beam at every mirror, So reusing the the stock mounts or finding some nice printable ones would be cool. So it all comes down to your setup, each one would be pretty specific.
As for accuracy, I agree with you, as is it is good enough for most but if you were really trying to push some resolution boundaries using stainless steel rails would be all you would need.