Help me
 

My baby, an 85 300zx NA, I turn to start the car, she has a slow crank, not too slow but slow sounding like it was the battery, I had just checked the battery all is good, I even put an alternator in 2 months ago and that reads good. could that be the starter ? the selenoid ? Is the starter very hard to change in that type of car, I can't find it, on my 82 280zx it's like right there .

Could be the connections, could be the starter. Doubtful it's the solenoid, but the solenoid is built into the starter itself, kinda.

is that hard to change the starter

Check the battery cables all the way down. Sometimes they corrode internally. If there are brittle spots, you know you have a cable problem. Like AZ_ZBum wrote though... Could be the connectors themselves. Finally, small chance but... There is a starter wire under the car that can get loose or dirty and cause a weak connection. I doubt that's it though.

Rod.

On my 1985 300zx turbo the starter was fairly easy to remove if your under the car it is on the right side a couple bolts and wire removal and it's out.

If you get that out you can take it to O'reily and they have a machine to test them on for FREE!