I rather expected that the car would be RHD. Given that, the M102 may be easier to install. Is the car currently equipped with manual transmission? If so, both 4 and 5 speed manual transmissions are available for the M102.

A carburetor equipped M102 would simplify the fuel system. M102 engines were supplied with carburetors outside the USA (which is where I am located). With respect to the matter of installing fuel injection, it is not just a pump change/addition, but it also requires that all the fuel line connections be upgraded to withstand pressure of about 5 bar, as well as the wiring and Engine Control Unit (ECU) required to operate the mixture control.

With respect to the M123 Solex carburetor, its absence is no great loss. Almost any other carb will be a substantial improvement. The M123 engine installation would require that the braces between the firewall and the inner wheel wells be removed; the cam box of the M123 interferes with those braces.