What's the height of the top of your garage door? A boat on a trailer is higher/taller than you'd think, and if you go for a boat with a cuddy and radar arch you may find you run out of headroom. A rib or open boat may still fit.
As for size of boat, I started off with a 4.4m cuddy boat, then quickly went up to a 5m cuddy, then 8m wheelhouse. Bigger is usually better when it comes to boats - more stable, better sea-keeping, can take more weather etc. Go for the biggest you can afford and you'll likely get more enjoyment out of it, so you'll want to keep it longer. You'll also have an easier time selling a mid-sized boat than a small one in my view as there's more of a market for them. By mid-sized I mean 16.5ft/5m trailer boats such as a Warrior 165.
As for location, a boat (vs a yak) gives you the range and speed to fish new marks and marks further away. Also means you can stay out longer and you'll be less tired without the paddling. It'll also give you a wider range of weather conditions you can comfortably fish in. So logically I'd argue that a boat should see you catch more fish simply because it gives you more fishing time compared to a kayak.
As for trailering to the Skerries, there aren't any decent slips nearby (that I know of). The one at Torcross lets onto shingle so you'll need a tractor or serious 4x4 to get the boat wet. Hallsands/Beesands are beach launches too. The public slip by the Higher Ferry at Dartmouth is probably your best bet, but the parking is nightmarish, and parking with a trailer nigh on impossible.