I know many like the swim platform but the OB has many advantages, lighter, less maintenance, no gas vapor in the bilge concern, much easier to winterize, etc. The 4.3 is a good engine power wise for an 18 footer but I hate the molded in fiberglass all around the rear of the engine, it makes certain maintenance jobs much harder than they need to be. Anything on the rear of the engine, exhaust manifolds, starter, steering actuator, power steering cooler, it will take the mechanic hrs to get at it. And that will cost you.
A 150 OB on that boat will make it fly, the 4.3 has 50 more hp but weighs 400 lbs more. Winterizing is flush the motor, change the engine oil, lower unit oil, fog the motor done. No crawling around in the bilge looking for drain plugs, straining to reach oil filters, etc. My boat has excellent access but it gets old, and there's a reason why IOs are expensive to keep.
And as far as hull color, if you are going to keep the boat a long time and can't store it inside, stay far away from colored gelcoats esp red and blue. They oxidize after a number of years and look awful, they take hours with the buffer to look good, but it does not last. I went through this with my 88 200 and I wound up getting it painted. A white boat maintenance wise is so much easier.