The Topaz went through two generations. The first, which lasted from 1984 to 1987, came with an I4 engine with a Mazda RF diesel I4 offered in some markets. The transmission was a 3 or 4 speed automatic or 5 speed manual, which was standard for Ford / Mercury compact cars from that era.
The second generation, from 1988 to 1994, came with an optional Vulcan V6 in addition to the standard I4. The 4 speed automatic transmission was dropped, leaving a 3 speed automatic or 5 speed manual as the only choices.
There were many trim levels for the Topaz. They included the base L, which was sold from 1988 to 1991; the GS, which was the mid-range option and most common; the GS-AWD from 1987 to 1991, which offered all wheel drive; the LS, which was the luxury high-end model;
the LS-AWD, which was the luxury model with all wheel drive; the XR5, which was the performance sports model from 1988 to 1992 and got the V6 engine; and the LTS, which was a 4 door version of the XR5 (the LTS stood for Luxury Touring Sedan).
These cars are fairly common. If you want to buy one, I advise against buying one from a used car seller. They tend to have higher prices than places like eBay, even if you have to factor in the shipping charges for moving a car a long distance. Chances are you can find one on eBay that is close enough to pick up. I'd rather spend a day traveling to get a car than pay a used car dealer $800-$1,000 more. There are also lots of parts on eBay that sell for a lot less than the auto parts store sells them.