Is the SRPK18 a good first automatic watch?

Yes. It has a proven Seiko automatic movement, day-date and 100 m water resistance, so it covers daily wear without much fuss. The main compromises are mineral glass and a basic bracelet.

Does the SRPK18 have a sapphire crystal?

No. It uses mineral glass, which is more prone to scratching than sapphire. Keep it away from abrasive surfaces or plan on a polish or replacement later.

How long is the power reserve?

About 41 hours when fully wound. That means it can sit off the wrist for a day and a half and still run.

It is rated to 100 m, which covers swimming and snorkeling. Avoid pressing the crown underwater and skip deep diving.

Can I put a different strap on it?

Yes. The lug width is about 22 mm, so most aftermarket straps and bracelets in that size will fit.

How does it compare with the SRPD Seiko 5 models?

It uses the same 4R36 movement and similar case size, so the difference comes down to dial and finish. The SRPK18's gold-tone look is the main draw over the steel SRPD versions.