Plenty of people use a 55 gallon (tall) for adults and they are happy & healthy, although it's not an ideal tank. The ideal tank would be a 4'x2'x2' tank (a 75 gallon long/breeder) or a 40 gallon long/breeder is minimum (which measures apx. 3'x1.5'x1.5', I think). So, if an adult can live in a 40 breeder, I think a 55 gallon would be just fine, too.
If you are getting a young beardie, you may want to block off some of the tank until he gets a bit larger to help ease any relocation stress he may have. Then, keep you eye out for a good deal on a better tank, either in a pet store that may be having a good sale or craigslist. CL always has great deals, especially if you live in a big metro area. Or, you can build your own enclosure, many think this is the very best option.
As far as substrates go: avoid particle substrates (some say you can use sifted playsand once they are adults, but I will avoid it). Even the calcium sand that says is safe, is NOT SAFE! You can use paper towels, newspaper, non-adhesive shelf liner, or tile (which is the best IMO).
Hope that helps!