To the poster above.
I drive a 1988 mustang with a Merkur engine swap and it's been nothing but reliable. What makes it break for people is the fact the engine hasn't been maintained.
My engine has 95K miles on it, and the valves look new, cross hatching inside the cylinder walls, and the cam looks new. I beat the crap out of the engine everyday (6000+ RPM daily) and even with a blown turbo, failed 3rd and 4th gear, and a bad FPR it still drove me home.
If you maintain it, it will be fine.
I totally agree with the last post on maintaining the engine. The durability of my 89 was incredible. I put 160k tortured racing miles on that car and it was still running, but had some blow by and needed a rebuild. However, unless you can pay a fortune, you have to be willing to do your own work.. because as the vehicles age, you will be replacing half the car, either by looking for salvaged parts, or reman, or new through the car clubs and websites. There is still plenty of parts availability and interest in these cars. But anything that old is not a daily driver without a lot of work.
FYI.. If anyone puts a Mustang t-5 tranny in as I did in the car... make sure you use a pilot bearing, not bushing, or the vibration noise will be very annoying.
42000 total roughly, I know of my year 87 they only made 7,321. They are rare, there is not even 1000 per state, and considering all the ones that have been decommissioned by accidents and fires, and blown engines and neglect, we're looking at maybe 20000 total survived to now, maybe more or less, who knows. In 25 years, a lot can be incapacitated.
Of course, they have very close cousins, the Mustang SVO's, and Thunderbird coupes were common cars here in America, and then there is the Sierra in Europe, which many many many were made. But the XR4ti themselves, only 42000 total.