For a long time the only aftermarket option (in the U.S. at least) was the one made by ANSA. On occasion you could also get a Stebro. These two are still around, and of course places like IAP and Centerline may carry other lesser known manufacturers. The stainless steel Magnaflow is the recent addition, and it was the result of work done by Simon, who is one of the founders of this BB. When considering an exhaust, I think it is crucial that you measure the diameter of your stock pipe, and ask about the dimensions of the ones you are considering. A slightly larger diameter exhaust will work very nicely, while a much larger one will not, unless you increase your engine's output. Off the shelf stocks are not going to be too large. so this shouldn't be a problem, but just ask if you are getting stock diameter or a little larger.
There are a lot choices on convertible tops, and a lot of different prices. I favor the fit of the Elektron top (~ 450 for a canvas, and ~250 for a vinyl), but there are plenty of others that look fine (such as Robbins, Topsonline EZ Softop, etc.) and are cheaper, and the IAP tops are also fine tops... (If you are considering Caribou, I would strongly recommend that you do a search on this BB, to understand some of the issues with their business reputation. That said, I have seen their tops and they look fine.) As far as vinyl or canvas, I think that there is nothing wrong with either. However, if I see a Spider that was originally fitted with a canvas top, and it is now fitted with a vinyl one, I perceive the Spider as having been "de-valued", especially since a top is something that will last years and years, and hence the price difference becomes a small factor over time...
As far as seats, there are lots and lots of places that supply the kits for very reasonable prices, as long as you are looking for a pretty standard color... If you are looking for the brown leather that was OEM on some of the '82 - '85 Spider Veloces, then be prepared to spend - brace yourself - $1200 or more to replace these. (Re-originals in the U.S. can get these, or you could contact Alfaholics in the U.K.) Another option would be to have a local upholsterer order the leather in the custom color and make the covers, since the pattern is simple for any good upholsterer to duplicate.
I would also encourage you to search this BB for past threads on the three topics that you are asking about, as there are a lot of threads that cover these in detail.