First, congrats on the new car!! to ans. some of your Qs
1- I don't think there should be rattling from the rear if you need shocks; any shade tree mechanic knows you push down on a part of the car to see how it rebounds to test shocks! Most likely a bad spring seat or something loose; but w/only 57K miles springs & shocks should be "NEW"
2- At a min. if the shocks are worn, replace both rears at the same time; if u can afford do front and rear. Also, "BEST" place to buy any/most parts is online; just know exactly what you want(get part #s if u can) and check for best prices or free ship(eg. oemparts.com;ipdusa.com;volvopartsworld.com). ALSO you must check out WWW.SWEDESPEED.COM for all the info you could need about VOLVOS.
3-tires depend on what you want from your ride: if you live if multi-temp area, get all-seasons V rated tires. If u want pure performance get Z-rated performance tires. bottom line-you can't beat tirerack.com for price & selection(check them out)
4-these cars are known to let a little oil at the turbine; usually a bad seal or something minor. I don't recall mine haveing to "take apart anything". BTW, is this mech a VOLVO specialist or the dealer? Try to find a VERY good volvo indie. tech and your ownership will be happy! Oh, if u can afford to, get an Extended Warranty for any major repairs that may pop up; good luck!!