Jeong-to answer your Qs:
*You don't need to modify the dash to accept the SC-901; it slides right in!
The amp.(a 4x50 or 4x60[if u have a S/V70]) goes behind the radio(850) or under the passenger seat.
*If u going to use an sc-901, you CAN'T go with an aftmkt. amp to power the radio. U have to use a Volvo amp($250-500) and get the wire-harness kit($90). Don't even bother trying to work around it, cause I tried and the effort is not worth it.(note: if you know a good volvo dismantler, you can get the parts for cheap) The $500 amp is a 4x100w unit that comes in the C70.
*I will have to check the invoice for the part#, but I recall the cost is $89+tax in NY for the wiring kit. Note: there is a bracket kit, but I passed on it since I wanted my installer to custom install it.
*The 901 is a bit hard to find but not impossible; ebay have them sometimes. I got mine from Cherry-autos.com in Ohio. It was a bit blemished, thats the main reason why I traded it for a "new" one. It was $250+ shipping(then I paid another $250 to get the "new" one).
My suggestion is, for much better sound you could change the door speakers to some 2-3 ways. Next get the 901 & amp. This should be about $800 is you shop wisely. $1000 if you goto the dealer for the amp, wiring and bracket. I used my tax refund, so it was money I was expecting. I went over my initial budget but I installed an amp and sub in the trunk. good luck, BD