89-93 5.0L Mustang will be one of the cheapest (using used manifolds, fuel rails, throttle body, MAF......). I found all of the used parts I needed on Ebay. And, bought new sensors at the local auto parts store and a new RJM harness.
If the motor is near-stock, the stock calibration might be 'close' enough. Tuning options are 'internet' tune $350, 'tuned' aftermarket MAF sensor and bigger injectors $350 or a custom tune / dyno session $$.
I don't need horsepower for the type of driving I do. I have a very mild 347 with stock EVERYTHING 5.0L '93 Mustang EFI. Wide band 02 data is acceptable.
$1500 for everything.