I'm no expert but I have some time in the Coast Guard on RHIBs and will try to help you out a bit.
The boat length is not going to effect dryness of the boat in my opinion, unless you're really giving 'er in the chop and even then water and weather conditions are going to be the biggest variables. Most fishing boats aren't meant for brutal conditions anyway and probably due to comfort you'll find yourself fishing in good water conditions anyway. Boat length will probably help with getting through the chop when you're taking it head on. A shorter boat, in my opinion, will find itself in the troughs more and is more apt to stuffing the nose in the oncoming wave on the way down. But I'm not sure how much a difference the 2' between a 16' and an 18' will really make. A V-shaped hull should help more in choppier water, it will sit lower in the water and be more stable. I think this is why your deep-water walleye boats, for example, are a V-shaped hull, whereas your bass boats don't draw as much in the water and have low decks because they're meant for the shallows. A 16' should be a bit more maneuverable than an 18' in the docks and in tight spots.
Windscreen is really only useful for the ride to and from your fishin' holes. It's a nice feature that you probably will never need when actually fishing. There are times in the rain or when the bugs are heavy that you will wish you had a windshield of some sort. The rain can hurt like a bitch at high speeds and the bugs are just annoying. I always wore a cheap pair of goggles on our boats.
As for the engine, the manufacturer should list the HP and/or weight range each hull can handle. There are also hull speeds that have to be considered. If it's the motor that came with the boat, from the dealer/manufacturer, it's likely fine. If not, you'd want to look into it further IMO.
If you're going to buy used the biggest thing is probably how the boat has been maintained. Things to consider would be the hull itself, and if it's a steering wheel boat you have all the cables to consider. There's a lot more that can go wrong while you're out on the water with a steering wheel boat than with a simple tiller boat. The biggest thing to consider with used would be the motor. I'm not experienced with this but if I were buying used and spending a decent chunk of cash I'd be asking about the motor, asking for receipts/logs, asking for the hours on it, maybe having a mechanic look it over. Often when buying used you may not be able to start up the motor and check it out because it won't be in the water. Motors are expensive. Trailers are another thing that have probably not been maintained as well with used boats, I would make sure the tires are in good shape and the brake and signal lights are in working order. All of this is stuff you don't have to worry about if you were buying new. You may buy used for what you think is a good price only to find that there is actually quite a bit of money that needs to be put into the boat upon further inspection.
Consider the cost of all the safety gear you need as well. Depending on boat type and size you will also need things like fire extinguishers, paddles, life jackets, bailers, a VHF radio (you legally have to be monitoring channel 16), etc. You can find a list of the necessary (and legally mandatory) safety gear online. Make sure you know the "rules of the road" so to speak, and can navigate through channels, etc. A depth finder will help you keep the boat in enough water. Then there is insurance, storage in the off-season, a vehicle that can tow it, etc. If you ever get in trouble out on the water call *16 on your cell and the CG will come get ya.
Hope I haven't over-simplified things and that this all helps.
