I'd imagine there is a credit system for personal use of resources in Starfleet. As for buying the house on Bajor well Bajor isn't a Federation World, and they haven't gotten to the point where they have unlimited resources for their people. One of my favorite episodes from season 1 called progress is basically the Bajorian version of eminent domain to gain farmland. So, they might still be using currency.
As for Raffi what can I say unlike most 24th Century humans she's a petty bitch.
As for why the Miranda was still in use vs the Constitution my guess would be the Miranda had many variants like the Soyuz Class which made it a versatile ship unlike the Constitution which was replaced by the more advance Excelsior Class.