Not a good place to start IMHO. If the fuel pump does work, and if the tank is full of crap/crud, then you'll be pushing all that crap/crud into other fuel system components, and possibly damaging them.
Besides, you already know that the engine turns by hand, thus the only reason to attempt to crank it now is confirm whether the starter motor and circuit are functional, which I personally wouldn't worry about until you have the tank drained, assessed, cleaned, and refilled with good/fresh fuel. You'll also want to replace any in-tank components that are bad.
If the fuel in the tank is really bad, then your next step would be to change the fuel filter and accumulator.
After that, you'd want to pull the injectors, put them in jars, jump the RPM relay (or apply 12V to the fuel pump so it circulates) and assess the state/condition of the FD/injectors, as well as baseline the CO/AFR setting on the mixture unit if it appears to be significantly off.
Once you've done all that, then I'd move onto to trying to crank it and get the engine started.
Start at the tank - get that situated first. Then work your way towards cranking it over and getting it started. You'll be glad you did.
Best luck