It is indeed going to be okay, you won't be the 1st who've successfully upgraded a set of wheels on a boxster.
The heavier the wheels are = less performance out of the car + heavier the stress is on the suspension assembly and wheel bearing.
Does look better with 18 though
Luck with finding the right wheels!
PS: avoid 'adapters' unless sourced from a well known engineering backed company/manufacturer. You are looking at around $300~500 for a half decent set of adapters (machined and heat/hardened 7075AL is not cheap). They are also quite rare and often made-to-order.
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