There are many members of the Moped Army who rave about Minarelli engines. Evidently they have good cause to do so. I don't know much about any moped but my own, but I have had very little difficulty working on its Minarelli engine, nor have I had trouble obtaining parts at a reasonable price.
The leak is almost certainly fixable. At the very worst, you could replace the entire Dell'Orto carb for a relatively modest investment. My carb was leaking as well, but I replaced it because I wanted to install a larger one ($15 for a rebuilt). The leak would have been easy to fix had I wanted to do so.
I don't know anything about the Minarelli MOPED brand, but I imagine it's got many of the same generic components worn by many Italian machines, which means you should be fine. And the engine is standard fare in mopedland.
If you purchase this bike and have problems with it, it's because you got a worn out or neglected piece of equipment, not because you made a faulty decision regarding brand. If it runs well, go for it.