You can usually find specs for any boat with a good google search. Just type in the year, model, and the word "specs" and you should get your answers. Same goes for the trailer, but you can guess and get pretty close.
Always add about 500 lbs to the boat. For instance, my 18' Ebbtide I/O specs out at 2130 lbs dry. That means basically once they build the boat, drop in the 4-banger and outdrive, and add all the electrical, that's how much it weighs when it shipped new. But, add in oars, lifejackets, a full tank of gas, oil, fire extinguisher, stereo and speakers, battery(ies), fishing stuff, skis, ropes, etc, I just add 500 lbs to that. So my boat's trailering weight when I hitch up is likely 2700 lbs. My trailer is all aluminum and only weighs about 800 lbs. I comparable steel trailer might be more like 1200.
So, to be safe, I have guessed that my whole rig maxes out at around 3500 lbs. That would be the most it could really weigh if I load it for a summer of boating.
Go big. Estimate high. Then be realistic on tow rigs. Factory tow ratings are a complete joke in some situations. Full size trucks are relatively accurate, but smaller vehicle tow ratings are highly based on warranty, market use, etc. Good example that I use a lot: 96 Caprice with 2.73 gears and a 4.3L baby LT1 is rated to tow 3500 lbs. Impala SS with 3.08s, extra oil cooler, power steering cooler, stiffer suspension, posi rear, and a 5.7L LT1 is only rated to tow 2000. GM figured people wouldn't buy Impalas for towing so derating the tow capacity was a calculated choice to save on liability and warranty claims.
My buddy bought a Rover Disco to tow because it said it could do something like 4500 lbs, but the floppy rear air suspension and short wheelbase made towing more than about 3000 a complete joke. Then of course his Jatco transmission blew up at about 40k and the nikasil engine cylinder liners came loose shortly after, but that wasn't the trailer's fault. That's just a Disco.
More wheelbase is always good. Having a heavier vehicle is good to a point. Towing 4500 lbs with a 3200-lb tow rig means that it controls you, not the other way around. Don't stress about power, but do stress about a transmission with a track record of being up to the task. I have loaded a 26,000 lb box truck to above capacity while towing a 10,000 lb trailer with a 180 hp Cummins ISB. It did just fine. A wee bit slow in the Rockies, but not like it was a danger. I was still passing semis. In that case I had a 6-speed Eaton Road Ranger box that probably could have towed a mountain.
Do give some consideration to brakes. Most vehicles these days (now that 19" wheels are the new 15") have ample room for brakes and OEMs do a pretty good job of stuffing good sized rotors and calipers in there. It doesn't hurt to give a nod to a vehicle with larger diameter rotors.
