I don’t have any direct experience with the get of MDO. But I know several sport horse people who I respect who “seek” them out.
As to “they do have a reputation for breathing issues, but the ones I’ve known have also been unsound”
Being in the TB business I am privy to “inside information” long before it hits the “street”. When “negative information” does hit the street the market for a stallions foals will drop like rock.
Given the fact the MDO stands for $125,000 and his yearlings average over $300,000 for the last couple of years I doubt there is reason to believe he is “known” for getting horses with breathing and or soundness issues.
What needs to be kept in perspective by those who buy TBs in the “secondary market” especially those who buy at the bottom and or get one for free is the fact these are not the “cream of the crop”. Regardless of how they preformed on the racetrack. So they are not a “true” representation of a stallion and or mare’s over all “quality”.
Unlike warmblood breeders which is tightly “controled” and only the best by and large make it to the “open market”. TBs are not “culled” and “disposed of” that have “issues”. There is almost always someone willing to take a “shot”. When these horses end up in the secondary market people have a tendency to “knock” the breed and or “blood” based on very limited experience. Social Media “spreads” these perceived “knocks” like wild fire. It is what it is. I could spend all of my little free time on various TB forums correcting misinformation and or presenting the “big picture”.
This is a link to a full sister to the OP’s horse;
http://www.fasigtipton.com/catalogs/2014/0209/581.pdf
The filly was a 2 year old and sold in a “mixed” sale for only $33,000. Obviously the filly had issues. Other wise she would not have been a “cast off” by Darley. Darely as a rule doesn’t breed for the yearling market or the 2 year old in training market. It is rare to see a young horse of theirs to be offered at a public auction. If it is you can pretty much guarantee there are some sort of issues going on. But I am talking about the racing/breeding side of things.
It looks like this mare, Interior Design is not a good “fit” with MDO for racing.
There is no reason to think the OPs horse will not be perfectly suitable for sport/pleasure. Well bred horses are usually in the hands of the best of the best. If they don’t pan out on the race track they pretty much never end up on “Canter”. Their trainer and or racing managers have some “go to” people that they will call to find them second homes. They are not looking to hide anything and or “squeeze” a few more dollars out of the horse. They only deal with people who they know and trust and the horse won’t end up in an “embarrassing” situation that will come back and “bite them”.
not saying it to be judgemental.