I just googled the C2 - the first random site I found is showing the tow weight unbraked as 450Kg, braked 530Kg (CHECK THIS IS CORRECT FOR YOUR MODEL!!!)
So, assuming an unbraked trailer is approx 150KG (light boat - light trailer) that gives you about 300 Kg to play with. I have a copy of the Tohatsu brchure on my PC, so I'll grab that as an example - the 4 stroke 25 is about 70Kg (shared lump with the 30, so slightly overweight for it's Hp), leaving you 230Kg for boat & fuel (more if you get a lighter 2- stroke)
A very quick filter of the spreadsheet I put together when investigating much the same as you are now (i.e would it fit the garage & could I tow it?), and the following ribs could be on your shopping list as they are under 200Kg: If you go braked then there's a couple more pop out the woodwork, but you also end up with a heavier trailer, so in reality don't actually end up being able to tow much more in the way of boat. A lighter engine also adds a couple.
Make Model Length kg
Avon Adventure 450 4.5 190
Humber Assault 4.3 4.3 190
Avon Adventure 400 4.05 130
Avon SR4M 4.05 159
Humber Assault 4.0 4 175
Menai 480SR 4.83 183
Menai 430SR 4.26 145
zodiac Pro 7 man 4.2 157
Rib-X Explorer 450 4.5 150
Beware the Assault - posts here imply it's not too great in a chop. The Attaque (the one that does handle chop) is 225Kg for the 5m.
I'll now retire out the way of the inevitable flurry of posts this quick rekky might generate........