If you are building a boat with a flat stern you can stow it standing on its stern. Run a pulley to the trailing edge of your cabin roof and use a small trailer winch or block and tackle to pull the bow up into position. Kills your rearward view but will allow you to use the dink you already have and would be EASY to launch in pretty much any conditions esp if it really only weighs #100! As for your swim platform you can make it with a hinged center section to allow for those times you need to trim the outdrive completely out of the water.
If I build one of those designs I mentioned,it will come in under 100#.My outdrive will come thru the transom right in the center of the swim platform,maybe a little higher,in the down position.I am planning to build twin swim platforms that have angled sides to allow for the sweep of the outdrive.
As far as those fold up porta boats go, I'd consider them an item for a RV/tent camper that wants to be able to paddle around a lake at a campsite. The sterns are SUPER flimsy unless you build some bracing (essentially make a plywood cutout the same shape as the entire stern) and then its still not going to be something you'd want to have your trip's enjoyment depend on. 'Twould suck being off on a weeklong excursion and have one of the flexible seam/hinges tear or separate.
I have been getting mixed reviews from other people I have talked to about Porta Boats.I only plan to use a dink when I do the Loop and have to anchor or moor.With the shallow draft that my boat will have,I have no issues jumping the rail and wading to shore.I'll have to plan that around the tides.I may leave myself high and wet with a long swim back to the boat.