There are several companies that make aftermarket sunroofs for vehicles. ASC and a few others that I dont recall. Volvo does not make its own sunroof panels they are produced by another company that sells Volvo the panel kit. I believe Webasto produces the Volvo panels.

While dealers who would love to sell you an entirely new car will say its a "saftey issue" (which neither here not there since they themselves are not crash test engineers AKA a lie to get you to buy a new car) one has to consider this because the install is not completed at the factory. With a factory installed roof you get the same installation method that was used in the crash testing of the vehicle. Its hard to predict how a non factory roof would react in similar conditions because of the different installation methods. While you can buy a Webasto panel you won't be getting the same panel that they use on an OEM car because of the different installation methods involved with aftermarket vs OEM roofs.

In the factory the vehicle's roof is prepared with a molded lip that the roof panel slides up into to form the seal. With an after market panel this lip is not present and not possible to reproduce. Some roofs offer a plastic lip while others use rubber seals to provide sealing. My S70 has an OEM sunroof and it appears that they provide a molded lip but the water trap is actually plastic on the bottom (kinda cheap considering my Honda Accord had a full metal water trap). In the corners of most sunroof trays are holes that lead to tubes to drain any leakage from the roof. Most sunroof leaks are because of the blockage of these tubes. You can clean them out with compressed air if they become blocked in most cases.

On the average an aftermarket sunroof will set you back from $1700 to $2500. There are a LOT of variances depending on who puts in the sunroof. If you really want to put in the sunroof see if you can examine examples of the shop's work to determine if their workmanship is up to par.

Overall Your going to have enough troubles with the OEM components of the car to begin with. I would avoid putting a roof into the vehicle and perhaps look at another car all together. The S70 is now on the consumer reports "worst picks" list.