The whole thing boils down to whether this is exactly what you want, and whether it is the best one you can get for the money.
There are always more used cars people want to sell than people who want to buy them.
The main elements are time and money.
How much money can you spend?
How long can you wait?
Somewhere, at some time, someone will want to sell either that 2.6 manual 190, or a manual transmission will become avaialable and you can install it in the 190 you already have.
Surely somewhere in Europe that transmission exists, waiting for you to buy it.
On the other hand, an M3 (I assume this is a convertible) is a very sexy car, and I don't think $10,000 is that bad a price.
Good luck with whatever you do.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"