Should I go with the Pentax k70 with 18-135 kit lens or get the body only and buy the Pentax 18-135 separately?
Are the lenses the same or is the kit lens of lower quality?
Exact same lens. Generally, the bundled price is lower than buying separately. Reasons to buy the camera body only include:
A kit lens is called a kit lens because it is offered in a "kit" with a camera body. Both items can be bought separately, but the kit option is convenient for both purchaser and manufacturer. If you buy the items separately, you are buying the exact same items...and likely paying more.
Note that a kit lens does not automatically equate to a lesser-quality lens. Often, cheap bodies are offered with cheap lenses – because there are plenty of customers for whom price is the most significant consideration – and you get what you pay for. But higher-end bodies are often offered together with a higher-end kit lens.
Pentax has a reputation to maintain; they chose the 18mm through 135mm zoom kit lens after careful consideration. This camera sports an APS-C size image sensor (Advanced Photo System – Classic format). The imaging chip measures 15.6mm height by 23.5mm length. We fit lenses based on what focal length is “normal” based on size. “Normal” translates to not wide-angle, not telephoto. “Normal” occurs when a lens is mounted with a focal length that about equals the corner-to-corner measure (diagonal) which for this imaging chip is about 30mm. The realm of wide-angle is about 70% of “normal” or shorter = 20mm. Telephoto is starts at about 2X “normal” or longer = 60mm. Pentax has chosen a kit zoom lens that covers this range, however this lens is biased as it provides generously in the telephoto realm, but only provides a smidgen into the realm of wide-angle.
My general advice, start with the manufacturer’s selected kit lens. This range will give you an ample introduction as to the versatility of a zoom lens. As to quality, kit lenses often get a bad rap. However it is unlikely that a novice photographer will be able to detect any deficiency. You can buy zooms that cover a larger span. You can buy “primes” (fixed focal length) that work better in feeble light. There are lots of choices if you have deep pockets.
I say stay with the kit lens until you get some “stick” time under your belt