If you are left handed Spencerian and italic are so different it's like an entirely different language. To get a nice stroke with an Italic nib I have to write sideways and down. It's hard on the brain till you get comfortable with the flip. It doesn't have any sort of negative impact on Spencerian that I can write with a fairly normal overhand or underhand approach except for the amount of time that I can spend on a single style.
I don't know how long it takes to develop your first script to a point where it is natural. I'm not there yet. I suspect that each additional script should become easier as fine motor control and understanding of how the pen/ink works with the paper develops.
I'm out of 303's at the moment, 10 more on order, so I've been practicing with G-nibs. I am officially addicted to the 303 when it works and it teaches a really nice touch for the nib quickly.
I strongly suspect that it doesn't really matter what you select as your first script so much as learning how to practice effectively and consistently, and spending enough time examining exemplars of the script to get a feel for the variations that you can play with.
I think Engrosser's script is a bit unnatural in that it is designed for copperplate and touch-ups. Spencerian being designed for handwriting; spencerian, business hand, ladies hand and then the period of silly embellishment gives a very large area for study if you get bored.
If I had started with Copperplate I would probably have a similar bias for Engrosser's script

.
Either way I find improving my penmanship a pleasant meditation.