I’ve slipped boats and trailered and I would never go back to keeping one in the water again.
- The boat is always a mess
- You’re always wondering about it in bad weather.
- You think it will be easier because you can just drive to the marina and hop in, but you end up taking a lot of stuff out of the boat for fear of theft.
With mine sitting on the trailer in my garage it’s ready to go on whim, usually stocked with fishing stuff, tubes, wakeboard, snorkling gear etc. Just toss in a cooler, hook it up to the truck, and we’re off. The wife and I have launching and retreiving down to a < 5 minute process each way. We pay for a season pass at Niawanda to avoid the douchebags on jet skis who constantly play in the launch area at the free Isle View launch. I’m sure in your area there are both free and paid launches and sometimes it’s more than worth the money to pay for a season pass somewhere. The people looking to save every penny are the kind you don’t want to be around and they tend to be completely disrespectful and ignorant of any sort of launch etiquette.
In the size you’re looking at you really don’t need much for a tow vehicle. Small to mid sized SUV with 3500-5000 lbs should do it. 18 footer being closer to the 3500, while 22 getting closer to the 5k. If you must store it in the water the costs are usually pretty high and that money could be going toward boat gas. 
As has been said, boats are not good on gas. If you’re doing a lot of cruising and watersports you can burn through a tank in a full day on the water even in the size you’re looking at. I budget my boat gas by not doing the budget billing on my National Fuel bill. My tiny natural gas bill in the summer helps pay for my large gasoline bill for the boat. Plus a lot of times we’ll go out for a day and cruise to a good snorkling spot, spend a lot of time at anchor swimming and hanging out and all that time the engine isn’t running.
Overall costs aren’t bad. I do my own winterizing each year so that saves $300 or so. It’s not difficult, just a little time consuming. Do a search and I know I listed all the steps on some post. Maintenance is pretty much like a car except you can’t remove the plug to drain the oil since it would just go straight into the bilge. Pick up a $50 oil extractor to suck the oil out the dipstick along with a $10 set of muffs so you can run the boat in the driveway using a garden hose for the raw water cooling. Other than that you’ve got the lower unit fluid to change once a season (do it in the fall to make sure there’s no water in there that could freeze and crack something), plugs, and the raw water impeller. Unlike the closed system in cars boats are sucking water out of the lake/river so they tend to chew up impellers. For $30 or so and about 20 minutes to change it I wouldn’t let one go more than 2 years. Cheap insurance since you can’t exactly get out and walk if you break down.
Figure 100-200 to set yourself up with everything to be legal if the boat you buy doesn’t come with a coast guard package. Life jackets for anyone on board, flares, horn, throwable floatation device, manual bailing device (aka, a bucket), bilge rags for any oil spills, fire extinguisher, waterproof flashlight and probably some other things I’m forgetting because the checklist is in the boat buried under some ice next to the house.
Buy as much motor as you can afford. I have never met a boater who said “I wish I had a little less power”, but I’ve met a ton that say, “I wish I had a little more”. It really won’t make that big a difference on fuel because you’re going to be working a smaller motor that much harder to get the boat on plane, especially with a load. 18 footer a 4.3 will be fine, 20 I’d start thinking 5.0 as the minimum. In boats this size you won’t spend much time going WOT anyway because the waves from other boats will be sending you airborne at that speed. Where it’s nice is when you’ve got 6 people in the boat and you’re trying to drag 200+ pounds of person out of the water on skis or a wakeboard.
I’d stay away from jets drives unless you REALLY want to spend some money on gas. Sure, it’s fun tearing around like a big jet ski but they are horribily inefficient when compared to an outboard, sterndrive, inboard or v-drive.
Once you get one, pick up a subscription to trailer boats or some other boating mag to help you pass the time in the off season. When I got done clearing the driveway today I went over on the side of the house where I store the boat and just stood there longing for summer to come.
