I've added an aftermarket sunroof to an older car many years ago and was sorry afterwords. For one thing, it has to have a sun shade or the added sunlight will bother you. Another thing is that they eventually leak (wind, water) and no two aftermarket SRs are exactly the same so if you ever need parts in later years, finding them can be a bummer.
My 745T has a factory SR and I rarely use it, it doesn't leak water but does add to wind noise. I'd never order a car with it as an option nor will I install another one in a car that didn't come with it. The 4 windows provide enough air flow or the ACs on so I have no good reason to drive with it opened.
My $0.02 on SRs

http://www.edmunds.com/used/1990/mazda/miata/index.html

I believe the entire 850-SVV70 roof panel is different, also the headliner is different because there needs to be room to hide the sunroof mechanism. Also, if you have a wagon, there are all SORTS of interesting acoustical things that happen, when, say, the sunroof is open and someone opens a rear window!

-Punxsutawney Phil

'98 V70T5 5-speed-moonroof-IPD bars-Volvo strut tower brace-skidplate-V-1

97,000 mi

I added the Webasto in-built electric sliding/pop-up glass sunroof
to my Volvo. Looks OEM. I think they are Volvo OEM manf but not
sure. Installers all across the country. Check it out:

http://www.mysunroof.com/index.html

the headliner of the sunroof on a 850 is surprisingly frail and poorly designed...the motor failed on me under warranty...i have rattle that can't be really fixed..they've added as much tape as they could to have the cover at least slide...my fix? folded wad of letter size paper to a size of 2x1 jammed into the sunroff...