On my 15' Harpoon with only 3 stays, I put the vang in line with the jib halyard on the stemhead fitting, and tighten the 4:1 vang to put enough slack in the forestay to disconnect at the stemhead fitting, and then lower the mast aftwards. If I am trailering a short distance, I'll leave the shrouds (side stays) attached to the chain plates. For longer trips, or if it's going into storage, I will disconnect from the chainplates and secure all 3 stays to the mast.
I don't know how the boom is attached on a Mariner. On the Harpoon, the gooseneck is riveted to the mast, so I use a pushbutton quick release pin to attach the boom fork to the universal joint on the gooseneck. It originally came with a bolt and nylock nut, but I got tired of having to have 2 wrenches to rig. On the O'day, the gooseneck slides in the mast slot, so it just gets lifted out. If I were to look at most any other smaller boat gooseneck, I would probably feel it's pretty self explanatory, considering the experience I've had with different boats. Your mileage may vary.
There is great debate on towing with motor on the bracket. I feel if it's a fixed position bracket, it may be more secure than a lifting bracket. Significant consideration goes into the construction and strength of the transom, as well as the size and weight of the motor. On the Harpoon, with a fixed position bracket through bolted into the 2" thick hollow transom, and a 2 hp 2 stroke Suzuki, I have towed with the motor on, and it's probably ok. But I still went ahead and fabricated a motor mount to the winch stand on the trailer, just to be safe. If it's worth doing, it's worth over-doing!

The O'day has a somewhat wobbly aluminum 2 stroke lifting bracket, and a 5 hp 4 stroke, which is heavier than a 2 stroke. I do not trailer with that motor on the bracket, the motor goes into the back of the car. I do not want to have to replace that bracket any sooner than necessary!
As far as ratings, 2 popular racing handicap systems are Portsmouth Yardstick, and PHRF (Performance Handicap Racing Fleet.) In Portsmouth, bigger numbers equal a slower boat. I forget what the numbers are based on. In PHRF, I think it's based on actual times for a 1 mile course. Local PHRF fleets adjust ratings based on individual boat configs and such. I think PHRF starts at 20' long boats. Ah, there, Wikipedia states PHRF starts at 7 meters, and Portsmouth is more likely for dinghies:
http://en.wikipedia.org/wiki/Performance_Handicap_Racing_Fleet and
http://en.wikipedia.org/wiki/Portsmouth_handicap
Unless you're racing, a ratings handicap is mostly just a convenient way to figure if one boat is generally faster or slower than another.
That looks like a very nice boat. A little spit and polish
http://forums.sbo.sailboatowners.com/showthread.php?t=117266 and she should be very nice.
It does look like the roller furled headsail either has no UV protection, or if it does, it's white colored UV protective strips along the leech and foot of the sail. White protectant fabric has been shown to be less UV resistant than colored. I'd consider getting a Sunbrella or Weathermax fabric jib sock for it. On second thought, it looks to be a wire luff furler, in which case the jib halyard is used to hoist and tension the luff. So you'd need to have a second halyard to use a jib sock, or do something like lower the sail to put the sock on. To drop this mast, you will lower the furled jib with the halyard first, and remove it from the boat, then unhook the forestay and lower the mast. In general, I think wire luff furlers are good for small boats like dinghies, and allow you to set and strike the jib easily. However, I don't think they do a good job of reefing the jib, though I know people who do reef with wire luff furlers. Also, I'm not sure they do a very good job of setting a tight luff, though this is somewhat more a function of the stretchiness of your halyard line, and if you have a winch to put tension on the halyard.