As to the rattle - there are compartments below the trunk floor and on the sides - make sure nothing is loose in there befre you change the shocks. Make sure the spare is fastened down tightly. If one shock is shot, replace both sides or preferrably all four.
I have had great luck with Bridgestone tires on my various volvos and the prices at Costco (Price Club) are pretty good. What ever you do, be sure to have the tires changed at a place that is careful about torquing the wheel nuts properly. If they aren't done correctly you will end up with warped brake rotors in a few thousand miles.
By all means get the turbo leak fixed as soon as you possibly can - you didn't mention if it was on the intake or exhaust side but either can cause your car to run badly and wear oout very prematurely.

I guess all the other Qs were well answered but, I probably can help with these2:

3- I replaced my performance tires for touring tires. I've grown tired of the loud road noise of the peformance ones and found a substantial improvement in comfort and handling with the new set of Bridgestone Turanza's. Very significant improvements in rolling noise and, surprizingly, bumping noise to driving on the bad Detroit roads. Performance acceleration seemed not affected but, handling in wet and snow conditions were greatly improved. Durability seems to hold well (~15k miles to date) however, the old Cooper set lasted 35k miles, even with my usual brisky driving. Turanza's should be my recommendation to you.

4- Take the car to the dealer for any engine repairs. Your car is too new and that cost me only $43 when I replaced the Turbo seal (60k miles) with them.

yup. that leak is the oil return line seal. I've had mine go twice and i'm at 80K miles now. As someone else said, the seal is a couple of dollars and it should be around an hour of labor. If your indy mechanic is not familiar with the process and thinks it will take longer, than maybe a visit to the dealer for this one fix is warranted.

did you get a manual or automatic? If automatic, I suggest you get the tranny flushed and filled with synthetic immediately. Mine started slipping around 60K and found the problem was common among owners and what to do about it. It shifts like new again. I will do the flush and fill every 30K from now on.

As far as that noise in the rear, I have the same issue. I have chosen to ignore it. BUT, I did finally realize that I had a noise in the front as well. Living with the noise in the rear for so long sort of masked the noise in the front. Anyway, I replaced the swaybar endlinks and it made the car much quieter again. Now I just have the slight noise in the rear over bumps. Those front endlinks are another weak point and, from what I understand from other owners, is another 60K fix that usually needs doing. Very cheap and quick, so not a big deal.

I put Continental ContiExtremes on a few thousand miles ago and have been very happy so far. I had the Michelin Pilots on before these. I bought the car with 30K on it and drove it with those tires till about 75K, so they lasted quite a long time. May have even been original for all I know.

The http://www.volvospeed.com/bay13.htm is a great site to start
with. An oilleak at the oil return line on the turbo is a common
prblem. Figure a couple dollars in parts and one hour labor to fix.
Your mechanic should not have to open the turbo -- the turbo itself
is very reliable. In fact, I would not let him open it -- better to
get a rebuilt one from someone who rebuilds htem all the time (but
really, that probably is not needed). If he insists, have a dealer
check it out first.

My 98 had a thump when I went over small bumps. It was an
exhaust hanger banging the underbody. Tapping the exhaust pipe
after the catalytic converter with a rubber mallat would reproduce
the problem and I found where the hanger was hitting. Not likely
your problem but something to try.

I have Michelin Pilot XGT H4. This is the thhird set of tires
on my 98 T5 and the ones I like the best. Others were too harsh or
too noisy. No tires have lasted very long though. Figure 25K miles
out of a set if you don't drive it too hard. Also, I live in a
mild climate (no snow, little rain) so this tire works for me but
may not be the best for your location.

Noisy rear shock is not that common in the sedan. Some of the wagons
had issues with this. Try lowering the rear seat backs while you
drive to see if that helps to pinpoint it. Also make sure the spare is
bolted down tight and not making the noise.

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!!