SL newcomer asks for advice on the purchase of a 2015er

Hello! I’m thinking about buying a SL 400 from 2015. I’ve been driving BMW for years and I want to change. I’ve seen that there are engines with 306 hp and 336 hp. Where are the differences? What are your experiences about vulnerability, repairs, service, etc.? What service costs are coming to me depending on mileage? I’m also happy to take tips on the must-have equipment. Which models do you think are value-stable or does the value almost fall due to the facelift? I have seen that there are good 2015ers for just under 70,000,-. If you watch vehicles from 2012 these are still traded with up to 50,000,- and vehicles from 2010 still cost 35,000,-. Is this all realistic? Thanks in advance for your tips!