wealth is dealt with on page 91 DS-9 RPG core game book, it is an advantage, wealth +1 to +6 and is bought like any other advantage.
Wealth +1 advantage gives an annual income to the character of 1-500 bars of gold pressed latinum
wealth +6 advantage gives an annual income to the character of 500,000+ of gold pressed latinum
as others have said the Federation has progressed beyond material wealth (to a point, they still have a credit system AFAIK) and I think any advanced society would eventually go over to some power (as in watt/hour) based currency as a matter of course, but many other societies do still have currency based systems in place in the Trek universe.
Wealth won't really help much in most area's of the Federation (though I think there would be some parts that would still accept gold pressed latinum) and obviously there would be lots of places that have to accept Federation credits.
Though as shown replicators basically makes currency redundant, as long as you have power.
As far as buying starships, Spacedock deals with that, page 18, How much for the USS Prometheus? Again buy the advantage Vessel +2 to +5 and Resources +0 to +3 and they suggest combining these two advantages with the wealth advantage (so the character has the ability to support the upkeep of the vessel)
AKA-Dean
"I will never make excuses for who I am. It is the way I was born. I am a HUNTER. a BONE COLLECTOR."
Wave Man, the term "wave man" is the English translation of 'Ronin' (Japanese word) and literately translates to "wandering person" and in a modern context a WaveMan is one who is socially adrift or a SalaryMan who is between employers.